From the passage below, please extract the most recent country to join NATO.

Context: NATO is a military alliance of twenty-nine European and two North American countries that constitutes a system of collective defense. The process of joining the alliance is governed by Article 10 of the North Atlantic Treaty, which allows for the invitation of "other European States" only and by subsequent agreements. Countries wishing to join must meet certain requirements and complete a multi-step process involving political dialog and military integration. The accession process is overseen by the North Atlantic Council, NATO's governing body. NATO was formed in 1949 with twelve founding members and has added new members nine times. The first additions were Greece and Turkey in 1952. In May 1955, West Germany joined NATO, which was one of the conditions agreed to as part of the end of the country's occupation by France, the United Kingdom, and the United States, prompting the Soviet Union to form their own collective security alliance later that month. Following the end of the Franco regime, newly-democratic Spain chose to join NATO in 1982.

In 1990, the negotiators reached an agreement that a reunified Germany would be in NATO under West Germany's existing membership. The dissolution of the Soviet Union in 1991 led many former Warsaw Pact and post-Soviet states to initiate discussions about joining NATO. Poland, Hungary, and the Czech Republic became NATO members in 1999, amid much debate within NATO itself and Russian opposition. NATO then formalized the process of joining the organization with "Membership Action Plans", which aided the accession of seven Central and Eastern Europe countries shortly before the 2004 Istanbul summit: Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, and Slovenia. Two countries on the Adriatic Sea—Albania and Croatia—joined on 1 April 2009 before the 2009 Strasbourg–Kehl summit. The next member states to join NATO were Montenegro on 5 June 2017, North Macedonia on 27 March 2020 and Finland on 4 April 2023.

Russia's invasion of Ukraine prompted Finland and Sweden to apply for NATO membership in May 2022, and the ratification process for Sweden is in progress. Finland officially joined on 4 April 2023. Ukraine applied for NATO membership in September 2022 after Russia claimed to annex part of its territory. Two other states have formally informed NATO of their membership aspirations: Bosnia and Herzegovina and Georgia. Kosovo also aspires to join NATO. Joining the alliance is a debate topic in several other European countries outside the alliance, including Austria, Ireland, Malta, Moldova, and Serbia.

Answer: Russia's invasion of Ukraine prompted Finland and Sweden to apply for NATO membership in May 2022. Finland officially joined NATO on April 4, 2023.