Tuesday, authorities reported that a Russian-made missile struck a Polish village, killing two people. In response, Polish leaders said they were considering invoking Article 4 of NATO's founding treaty to compel the 30-nation alliance to take action.
Here's a rundown of what this treaty provision entails and what could happen after Tuesday.
Polish President Andrzej Duda stated in a televised address on Tuesday evening that it was highly likely that Poland would invoke Article 4 at a meeting on Wednesday after the missile entered Polish territory just five kilometers from the Ukrainian border.
The office of Polish Prime Minister Mateusz Morawiecki stated on Tuesday that it was in discussions with allies regarding the potential invocation of Article 4. Poland is a member of NATO, but Ukraine is not.
Article 4 permits NATO members to "consult" whenever one of them believes that "the territorial integrity, political independence or security of any of the Parties is threatened." However, it does not mandate or guarantee any type of action or coordinated defense response.
Several Eastern European nations triggered Article 4 at the start of the Ukraine conflict, as some of them share a border with the country. NATO held discussions with Ukraine and pledged financial and military support, despite the fact that member states did not send their own troops.
Polish authorities have not mentioned invoking NATO Article 5, which states that any "armed attack" against a NATO member "shall be considered an attack against them all" and is considered a cornerstone of the alliance.
Article 5's invocation does not require every ally to respond with military force. Instead, they can determine their own response. Article 5 has been invoked only once in NATO's history, following the September 11 terrorist attacks on the United States.
Tuesday, a Russian missile struck Poland, prompting several experts to predict that Article 4 will be invoked.
Ben Hodges, former commander-in-chief of U.S. Article 4 talks would be appropriate if the incident was not a deliberate attack on Poland, according to Army operations in Europe.