Article 4 of the NATO

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About Article 4 of the NATO:

If the missile’s origins are Russian, this is the first time since the start of the conflict in Ukraine in late February that Russian weaponry has impacted a NATO country.

It came into being after World War II as a counter to the Soviet Union’s possible expansion attempts in Europe. Then-US President Harry S Truman signed the 12-member treaty on April 4, 1949.
After the collapse of USSR in 1991, several eastern European nations previously members of the Soviet Union joined NATO. As of now, NATO comprise 30 members.

Under Article 4, any member state can convene a meeting of NATO members to “consult” when it feels its independence or security are threatened. It sends a strong political symbol to the greater world that NATO is concerned about a particular situation.
Article 5 is known as the “one-for-all and all-for-one” article. It states that an “armed attack” against one member is an attack against all and sets in motion the possibility of collective self-defense.
In theory, Articles 4 and 5 can only be invoked at the request of a NATO member. However, Article 5 has only been invoked once — immediately following the September 11, 2001, terrorist attacks against the United States.
Article 5 was not invoked when Russia invaded Ukraine, because Ukraine is not a NATO member. But Poland is a NATO member, so if an investigation found any evidence it was a deliberate attack, it could have invoked Article 5.