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Per title. E.g. right now the US has a mutual defense treaty with the Philippines and also with South Korea. If the Philippines and Korea go to war with each other, who is the US obligated to help?

I imagine this must have happened in history (making this perhaps better asked on the History SE), but I can't find any examples, and in that case this is likely the better place to ask. The closest example I found was US involvement in the Falklands war, but that never led anywhere because the US wasn't legally bound to assist either side:

At first glance, it appeared that the US had military treaty obligations to both parties in the war, bound to the UK as a member of the North Atlantic Treaty Organization (NATO) and to Argentina by the Inter-American Treaty of Reciprocal Assistance (the "Rio Pact"). However, the North Atlantic Treaty only obliges the signatories to give support if the attack occurs in Europe or North America north of the Tropic of Cancer, and the Rio Pact only obliges the US to intervene if one of the adherents to the treaty is attacked—the UK never attacked Argentina.

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    If it's 'mutual defence' the key will be who is the aggressor of course there is some leeway in how that is determined – user19831 Oct 31 '19 at 13:27
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    The tangle of military treaties is the reason that we went from a Serbian assassination of an Austro-Hungarian Duke to a World War that was largely blamed on Germany. The Web of honored treaties was so tangled that the exact progression from assasination to "War to End all Wars" is still not fully understood to this day. The entry of all great powers in Europe into the war turned what was likely a one year affair into a 4 year war (Austro-Hungary declared war on Serbia in 1914 and Serbia was defeated by 1915.). – hszmv Oct 31 '19 at 14:28
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    @phoog the US did not regard Britain as the aggressor as the US has never regarded the Falklands as part of Argentina. No LA nations went to war with the UK as would have been required under the Rio Pact, though some did express support. – gormadoc Oct 31 '19 at 21:25
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    You just choose one side. That one that is going to win. Vae victis, pacta non sunt semepr servanda. Italy in WW1 also just chose the side with a better promise of the booty. – Vladimir F Oct 31 '19 at 21:50
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    @gormadoc the thing about multilateral treaties is that it usually doesn't matter what one party's interpretation is. – phoog Nov 1 '19 at 0:16
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Very smart people are still trying to figure this out.

Perhaps surprisingly, this is far from resolved, and is currently an ongoing topic of rigourous legal and political discussion.

A good starting point might be Valentin Jeutner's book Irresolvable Norm Conflicts in International Law, based on his doctoral thesis defended at Cambridge University in 2015, which is the latest in a recent series of monographs that address norm conflicts in international law.

A fairly-involved review of the book discusses this specific topic and its current unresolvedness:

Egypt is party to both the regional 1950 Joint Defence Treaty and the bilateral 1979 Peace Treaty with Israel. Under the Joint Defence Treaty, a multilateral military alliance treaty with members of the Arab League, Egypt is under an obligation to aid any state party to the treaty in the case of aggression against that state, whereas the 1979 Peace Treaty with Israel puts Egypt under an obligation to refrain from any direct or indirect use of force against Israel. Should Israel now use force against an Arab League state and should that state invoke the Joint Defence Treaty, Egypt might find itself facing two potentially conflicting treaty obligations: to aid the Arab League state through the use of force against Israel and to refrain from the use of force against Israel.

[...]

The court will need to exercise an interpretatory function, and the interpretation will probably hinge upon the concrete question submitted to it. Therefore, it might not actually come to interpret obligations that are in conflict with each other. In order for a court to issue a dilemmatic declaration in the sense that Jeutner proposes, it would need to be the state facing the conflicting obligations to submit the conflict to the court. This seems rather unlikely in contentious proceedings.

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    @Sean: No. If you and I make a deal that includes my selling you my car, and then Roger and I make a deal that includes my selling him my car, I can't just go back to you and say that my obligation to you has now been overridden by my obligation to Roger. Rather, my deal with you should have precluded me from making the conflicting deal with anyone else. – ruakh Nov 1 '19 at 6:05
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    @Sean: For the same reason that crossing my fingers behind my back while "selling" my car doesn't mean that I get to keep both the money and the car. The 1979 Peace Treaty involved major concessions on Israel's part; Egypt's preexisting treaty obligations to other countries don't entitle it to accept those concessions from Israel while ignoring the promises it made in return. – ruakh Nov 1 '19 at 6:36
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    @Sean International treaties don't always follow logic. There can be conflicting treaties with different parties without either superseding the other. Treaties can be ignored if they are inconvenient. Depending on the incident, it might lead to international sanctions and internal political backlash, but if there is reasonable cause, like such a conflict, people might just accept it. Treaties are in essence just statements of intent and while most tend to be turned into law through ratification, they can't force a country to act against it's wishes if it's willing to accept consequences. – Morfildur Nov 1 '19 at 8:04
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    @Sean: "I can't have my cake and eat it too" is exactly the issue. I shouldn't have made two contradictory agreements with you and Roger. Having done so, I can't simply weasel out of my promise to one of you by saying it's overridden by my promise to the other. – ruakh Nov 1 '19 at 14:50
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    @ruakh note that neither treaty specifically precludes other. They are not in conflict as treaties, but only if specific unforeseeable conditions arise. – Gnudiff Nov 2 '19 at 6:23
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Most mutual defense treaties are very explicitly defense treaties. They cover the case where a third nation attacks one or both of the signatories. It is rather difficult for two countries to attack each other first. So the country which has a treaty with both has to see who was attacked and who did the attacking.

  • In theory, a straightforward matter of facts and international law (what is an act of war and what is a provocation short of war ...).
  • In practice, this judgement will be clouded by politics, and the treaty partner could deliberately take the wrong decision or get itself blinded by political bias.
  • A treaty partner of both might work to calm the situation instead of getting involved, too. Again a political decision.

For historical perspective, look at the various clashes between Greece and Turkey, both NATO members.

Of course there might be cases where a "defense" treaty also covers offensive action. While this example concerns just two parties, the relatively unconditional Imperial German support for Austria-Hungary is one of the contributing factors for the start of WWI.

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    @Allure which one performed the first act of war (Japan or Malaysia) on the other in your scenario? That tells you which is the aggressor and which is the defender. – Tim Nov 1 '19 at 1:30
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    @Tim right, but Korea and the Philippines would be at war without either of them being the aggressor. Their ally might have attacked, but not them. Roger's answer above gives a better example, as well. – Allure Nov 1 '19 at 1:39
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    @Allure oh you’re saying there’s a fifth country with a pact with both P and K? This is getting complex but I’d assume one follows the chain down? If J attacked then the new number 5 would side with M and P, and vice versa. – Tim Nov 1 '19 at 1:42
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    @Allure, chains of treaties are blamed for the tensions in the run-up to WWI. Austria has a quarrel with Serbia, and for that reason Germany violates Belgian neutrality to attack France? Completely logical if there is a tangled web of treaties .... – o.m. Nov 1 '19 at 5:50
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    It is rather difficult for two countries to attack each other first. — it is rather easy for two countries to accuse each other of attacking first, such as at the start of WW II or the Korean War. – gerrit Nov 1 '19 at 9:02
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In your situation, assuming de-escalation didn't work, the country in the middle would do one thing, and one thing only: whatever is in their best interest.

The main signpost that helps political realism to find it's way through the landscape of international politics is the concept of interest defined in terms of power.

...

Intellectually the political realist maintains the autonomy of the political sphere, as the economist, the lawyer, the moralist maintains theirs. He think of interest defined as power as the economist thinks of interest in terms of wealth...The political realist asks "How does this policy affect the power of the nation?"

This could mean taking no side, taking a side that has longer upside potential, selling arms to both, or any myriad of policy positions. But these positions will not be based upon whatever is stated in some treaty.

All quotes from Politics Among Nations, Hans Morgenthau, sixth edition

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    This is probably the most realistic answer, judging from international examples. One of the latest examples (not explicitly a defense treaty, but covered similar topics) would be the major nuclear powers that signed the Budapest memorandum en.m.wikipedia.org/wiki/… guaranteeing the territorial integrity of Ukraine. We know how that turned out. – Gnudiff Nov 2 '19 at 10:06
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A decision to adhere to a treaty is always a political decision.

It is a mistake to think about international treaties as if they were laws, because there is no international government with the mandate and capability to enforce them. If you and I make a contract, and I break it, you can go to court to make me honour it. There is no equivalent for international treaties. (Yes there are international courts, and the United Nations, but neither is in a position to enforce its decisions unless a majority of powerful nations has the political will to back them. Meaning it's still a political decision.)

Therefore each country decides if it will follow up with its treaty obligations when it believes it is in its interests to do so. That's not to say they look only at the short term convenience - most countries don't want to get a reputation for breaking their treaty obligations (though one major country doesn't seem to care at the moment), and there may be punitive repercussions for doing so, but it is about balancing the political effects of following the treaty with the effects of breaking it. If there is a grey area (such as some ambiguity about who attacked who) that often gives countries leeway for deciding whether or not to take action. It depends on the national and international political effects.

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tl;dr: The side that wins the war is the one that was in the right.

In order to invoke the defense treaty, the attacking side will claim to be a defender, because they want the third party to be on their side, or at least to remain neutral.

Say the Philippines dress up a small group of operatives in Polish South Korean uniforms to attack a Philippine radio station. They then claim to have been attacked, and start continue the war in self defense.

Let's assume the US ends up joining the Philippines.

If they end up winning the war, they will make South Korea claim responsibility for the original attack on the radio station, and all is well.

If they lose the war, South Korea will make the Philippines claim responsibility for the attack and demand some kind of reparations from the US for breaking the treaty - the kind of reparations doesn't depend on the wording of the defense treaty as much as it does on the kind of relationship they want to have with the US going forward, and the sum of the military, economic, and political leverage they have. For example they might agree to drop any claims they have in return for the US changing sides mid-war, or they could sink the entire US Navy, invade the US, and instate a puppet government that agrees to pay monetary tribute to the Philippines for 50 years.

If there's a stalemate, everything has to be sorted out in one huge mess.

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  • This is the answer. A treaty is only as good as someone's ability to enforce it. The victor of the conflict will be the one to enforce it and exact penalties. If a country has conflicting treaty obligations, they will want to pick a side - the side that is going to win. – Seth R Nov 1 '19 at 16:27
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This question seems to assume that military support is a binary thing and/or that a mutual defense treaty typically has some concrete military responses spelled out, so that only another treaty casts doubt on the response. In reality, mutual defense treaties have ambiguous enough response clauses so there can be many more factors (besides another, conflicting treaty) contributing to variations in [non]response.

To pick a concrete example, NATO's putative response to a possible Russian aggression has often been discussed lately. The following tables from an extensive 2019 RAND report outline these issues: it finds that variation in response from NATO members could be significant, depending on a number of factors, including how they perceive the threat, their domestic political situation, and perceived consequences for [non-]participation:

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