Thats why Article 5 (an attack on one is an attack on all) was not invoked for Iraq since it was an offensive war. It was invoked for the only time ever after the 9/11 attacks. Then the alliance went into Afghanistan, where half of the current NATO force is European (other half is American). Call it an offensive war if you want, but within the NATO treaty it is explicitly stated that if Article 5 is called, allies can respond how they want. Which is why some allies sent a token force to Afghanistan and some went as far as sending tanks. Its up to the political will of the government. Which is why Germany didnt contribute to the Libyan air campaign. That being said, if you are under protection of not just Article 5 but also the nuclear umbrella of NATO, some contribution is expected, at the very least in joint readiness exercises and local operations. For example, NATO just launched Operation Sea Guardian to help combat human trafficking in the Aegean and Greece begrudgingly sent a submarine. One would expect an eagerness to contribute to an operation that pertains so much to a local situation but the political will doesnt seem to be there.