NATO Day is observed worldwide every April 4 to commemorate the signing of the North Atlantic Treaty, which led to the establishment of the North Atlantic Treaty Organization (NATO) in 1949. This intergovernmental military alliance, which includes 30 European and 2 North American states as of March 2024, is based on the principle of collective security. Celebrations typically involve flag raising ceremonies and various events, although not all member states observe it on the same date; for example, Romania celebrates it on the first Sunday of April, while Lithuania commemorates it on March 29. The origins of NATO can be traced back to post-World War II alliances, including the Treaty of Dunkirk and the Treaty of Brussels. NATO has evolved to conduct military operations since the Cold War, with its first actions occurring in response to the Iraqi invasion of Kuwait in 1990.