The question of a military conflict between NATO member states would have been an entirely hypothetical question a decade ago , but looking at certain NATO member states (e.g. Turkey) increasing alliance with Russia , it just seems plausible that there can be a military conflict in between NATO member states.

Here's an example - Some months ago , Turkey came very close to a military conflict with U.S. forces in northern Syria.

Organisations like NATO or UN have been created to stop military conflicts. But what happens when a NATO member state declares war on another NATO member state?

From my yesterday's question about whether NATO can remove a member state from the organisation, I couldn't receive any positive answer on whether NATO can remove a member state.

So in case of war between NATO member states, NATO has no provision to remove a member state and then trigger article 5? So what side would NATO member states would take in such a case?

5 Answers 5


Although it's not explicit, it seems like all of the NATO members would be obligated to act in defense of whichever member was attacked.

Looking at the relevant parts of the North Atlantic treaty:

Article 3:

the Parties... by means of continuous and effective self-help and mutual aid, will maintain and develop their individual and collective capacity to resist armed attack.

Article 4:

The Parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened.

Article 5:

The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them... will assist the Party or Parties so attacked...

So, in theory, military conflicts among NATO members would essentially be resolved by the NATO members deciding who is the 'attacking' member in the conflict and defending the 'attacked' member. Whether this is what would actually happen is unknown, as there isn't an explicit mention of such an event in the treaty.

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    What exactly assistance is seems to have enough gray to allow countries to effectively stand aside, or even support the aggressor at least indirectly. I find it odd that article 9 doesn't come up.
    – user9389
    Commented Mar 9, 2018 at 17:05
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    @notstoreboughtdirt: True, although it's implied that the members will assist eachother, the phrasing "it deems necessary" and "shall recommend measures" basically means the NATO remembers can respond however they feel like.
    – Giter
    Commented Mar 9, 2018 at 18:03

If one member attacks another member of NATO, the aggressor is out of NATO via article 8. And then at war with all of NATO via article 5.

Now, you know what's the pinchy topic? Imagine Turkey attacks Cyprus (that is not in NATO but in the EU).

The EU treaty article 42, item 7. states there is a clause of mutual defense clause (if any member state gets aggressed all others must defend it), it has NATO exception in there stated (to comply with article 8 of NATO that was signed before), therefore the NATO members would not attack Turkey.

Specifically, article 42(7) TEU states:

If a Member State is the victim of armed aggression on its territory, the other Member States shall have towards it an obligation of aid and assistance by all the means in their power, in accordance with Article 51 of the United Nations Charter. This shall not prejudice the specific character of the security and defence policy of certain Member States.

Commitments and cooperation in this area shall be consistent with commitments under the North Atlantic Treaty Organisation, which, for those States which are members of it, remains the foundation of their collective defence and the forum for its implementation.

But technically the non-NATO EU members would have to defend Cyprus and attack Turkey....and the EU NATO members would have to defend Turkey because of NATO article 5, which would go into a loopy spiral like in the first world war.

  • Welcome to Politics! Which EU treaty are you referring to?
    – JJJ
    Commented Oct 15, 2021 at 1:13
  • The one currently in place. Treaty of Lisbon Commented Oct 16, 2021 at 3:01

In the specific case of a conflict with the United States, the opinion of the United States would be the deciding one.

While it may seem that this is solely because of political or historic reason, that's not actually the case. By NATO's charter, the supreme military commander of the NATO forces must be an American. And since a US President is the Commander-In-Chief of all US military forces, this would put the supreme military commander of NATO below US President in the chain of command.

In the unlikely scenario of a conflict between 2 NATO nations such that neither of those nations is the US, as members of a military alliance, NATO forces would have to follow the chain of command.

In case of a clear attack (as opposed to a misunderstanding or an accident), an attack on any NATO member is an attack on all NATO members. Which would put the attacker in a hypothetical state of civil war in which the attacking forces would not be able to request NATO assistance. But the chain of command would still remain as described above.


This is a hypothetical question. Article 8 of NATO's treaty says:

Each party declares that none of the international conflicts now in force between it and any other of the Parties or any third state is in conflict with the provisions of this Treaty and undertakes not to enter in any international engagement in conflict with the Treaty.

Hence the states are obliged not to act aggressively towards each other. Given the stability of NATO - it was established in 1949 but only came into force on the onset of the Korean War in 1951, one ought to dismiss this question. The only time the provisions came into force was in the immediate aftermath of 9-11. In the entirely hypothetical situation that a minor conflict occured between two participating members of NATO, I expect major diplomatic and economic pressure would resolve the issue. If not, then we will be in new territory not explored by the treaty itself.

The best example that illustrates this process is disputes between Greece and Turkey in 1974 over Cyprus where significant diplomatic and economic pressure was exerted on both parties to come to a workable solution. This prevented the situation becoming a full-scale conflict. It's not ideal, as Turkish Cyprus is only recognised by Turkey and a few other nations.


In case of an attack on a NATO member the other NATO members are obliged to help the attacked NATO member as per article 5 of the NATO treaty. In case of the attacking nation being a NATO member this probably means that the attacking nation would be immediately and effectively out of NATO the moment of the declaration of war on another NATO member even though there is no specific provision for that case in the NATO treaty.

NATO is a defensive alliance. The nation that any member of the NATO could get away (and stay NATO member) while attacking another NATO member seems not very realistic to me.

In cases where armed conflicts boil slowly and the question of attacker and defender (both members of NATO) is less clear I guess that NATO might have a problem and needs to clarify its foundation.

  • My guess is that ... - personal opinion is irrelevant here.
    – user17569
    Commented Mar 14, 2018 at 7:17
  • @anonymous You're right and it was more a figure of speech expressing general uncertainty not personal opinion. There is no specific provision in the NATO treaties for the conditions of a departure of a member, so different things are possible. Answer updated. Commented Mar 14, 2018 at 9:52