Military capability of Western alliance remains 'formidable' despite questions around untested 'mutual assistance' agreement as Trump pivots away from Europe

Article 4 allows member states to start a formal discussion within the alliance about threats to their security. It does not commit the alliance to military action.

Article 4 of the North Atlantic Treaty Organization is not as well known as its Article 5 collective security guarantee — which says an attack on all is an attack on one — but it is being increasingly invoked,

Ukraine conflict, NATO forces engaged in a direct defense of allied airspace overnight Tuesday-Wednesday. The incident occurred when approximately 19 Russian-controlled drones breached Polish airspace,

The largest group of European nations not in NATO are those that profess their neutrality. Including nations such as Austria, Ireland, and Switzerland, some have military neutrality enshrined in their constitutions, or because of previous dealings with the great powers.

Polish Prime Minister Donald Tusk said on Wednesday he had asked NATO to open consultations under Article 4 of the alliance's treaty after Poland shot down drones in its airspace following what he called a "large-scale provocation" by Russia.

Article 5 is an alliance-bound agreement between NATO nations in the case of an attack, as tensions between Russia and Poland increase.