What happens, from an international law point of view, if a NATO country which is not a member of EU, attacks (unprovoked) an EU country which is not a member of NATO?

  • Specifically, are the countries members of both EU and NATO legally allowed to intervene?
  • If they do (whether they are allowed or not), are the other NATO non-EU countries allowed to retaliate?

This is specifically from an international law point of view, what would actually likely happen is outside of the scope of this question.

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    Care to explain the downvote? The question seems perfectly legitimate and answerable to me.
    – o0'.
    Mar 18, 2021 at 16:39
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    If I'm understanding correctly, this question is asking about conflicting treaties, right? NATO countries are bound to defend other NATO countries, so are they allowed by that treaty to fight another NATO country, even if its to defend another of their allies?
    – divibisan
    Mar 18, 2021 at 16:39
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    Random example, just to clarify things, but please don't consider this to be part of the question: US (NATO only) attacks Ireland (EU only), what is Germany (both) allowed to do. Second part: if Germany decides to defend Ireland against US, what is Turkey (NATO only) required to do?
    – o0'.
    Mar 18, 2021 at 16:43
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    What happens? NATO would cease to exist.
    – Roland
    Mar 18, 2021 at 19:04
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    @JoeW the purpose is asking what would legally should happen, which is answerable. This is totally unrelated to what would happen (unanswerable) or what is speculated to be about to happen (irrelevant).
    – o0'.
    Mar 19, 2021 at 16:08

2 Answers 2


No (modern) international agreement condones wars of aggression. What article 5 of the NATO treaty and article 42 (7) of the TEU establish is an obligation to assist the country being attacked (with many nuances and caveats) and certainly not any obligation to help a country attack another one (even by remaining neutral). There is therefore no conflict of norms and no obligation towards the attacking country.

Both texts are written under the assumption that member states would respect international law and not attack each other (or anyone) and do not explicitely forbid providing assistance or really any military operations, they are only concerned with defense. As such, there is no need to be “allowed” to provide assistance, that's a given, and what these treaties create is an affirmative commitment to do it, under certain conditions.

Both texts also refer to article 51 of the UN Charter, which establishes a right to self-defence and foresees a quick involvement of the UN Security Council.

The retaliation question is a little more complicated. Neither texts spell out what should happen but it wouldn't make sense for an aggression to open a free for all. In general, “retaliation“ is also thought to be forbidden and assisting self-defence doesn't necessarily entail invading the attacking country (cf. the first Gulf war). It's difficult to see how an attack on the military forces of the attacking country could possibly trigger any self-defense clause or justify another assistance requirement.

In any case, the EU mutual defense clause is clearly subordinate to the NATO commitment:

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.

Conversely, article 8 of the NATO treaty provides that

Each Party declares that none of the international engagements 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 into any international engagement in conflict with this Treaty.

NATO countries therefore see their alliance as more important and framed their EU commitment accordingly. Incidentally, the EU mutual defense clause also contains another caveat to reaffirm the neutrality of several member states. In fact, the only non-NATO EU member state that does not maintain a policy of neutrality is Cyprus (which is denied NATO membership for obvious reasons).

  • 5
    The EU clause is much stronger (member states are required to render any aid they can) while NATO is just a request for help (where a postcard "stay strong" would be enough). Mar 18, 2021 at 21:03
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    @MartinSchröder I am not sure I agree. The EU clause has many caveats and doesn't even require military assistance. By contrast, the NATO treaty says that members will assist each other (the word request is not used in this context in the treaty, even if a request for help is obviously how this works in practice, both in the EU and NATO) and consider any attack as an attack on all of them (doesn't get stronger than that). NATO countries are also backing this up with a much stronger integration of their military apparatus.
    – Relaxed
    Mar 18, 2021 at 21:42
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    However I didn't write anything about that and I fail to see how this contradicts or otherwise concerns my answer. The EU mutual defense clause is explicitely subordinated to NATO commitments (in keeping with the NATO treaty itself). It's not the same as saying it is stronger or weaker. It could be even stronger or much more specific (as EU law often is), it could still be set aside entirely by NATO members. That's the meaning of the text (it's also quite clever and typical of EU compromises between diverging interests). Ditto for neutrality.
    – Relaxed
    Mar 18, 2021 at 21:48
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    While invading an attacking country may not in general be legal, it seems worth mentioning that in the First Gulf War the UN authorised the use of “all necessary means” to force Iraq out of Kuwait, which did give legal coverage to invasions of Iraq that were deemed to be necessary.
    – Mike Scott
    Mar 19, 2021 at 21:52
  • "Both texts are written under the assumption that member states would respect international law and attack" <- Well, that's certainly not true. NATO states routinely break international law by attacking and/or invading other world states with no legal cause. And EU NATO members take part in this. I suppose the assumption about not attacking each other is valid at least since 1945... although, there was the Balkan war, whose constituent states are now members (though they weren't at the time).
    – einpoklum
    Sep 2, 2021 at 8:08

Article 5 of the NATO treaty (the article covering mutual defense) covers when a member state is attacked. If a NATO member attacks another state without provocation, Article 5 does not apply.

  • Mutual defense covers both defending and not attacking? So, not only are they not obligated to defend or assist the NATO attacker, but they’re free to fight that country if they desire?
    – divibisan
    Mar 18, 2021 at 16:57
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    The problem is that since the end of WW2, states who agress other in wars makes sure their adversary look as guilty as possible. In other world, the agressor is almost always ambigious. Also, usually in modern war "rebel groups" are involved, and powerful nations makes war on 3rd world nations's soil through different "rebel groups".
    – Bregalad
    Mar 18, 2021 at 21:36
  • @Bregalad: How is that a "problem"? The practical outcome is that military actions require previous political action so that the military action will be seen by others as defensive. This political framing takes time, and allows for de-escalation. In the particular example, all other NATO and EU states would be de-escalating, and the political framing is bound to fail.
    – MSalters
    Mar 19, 2021 at 11:09

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