The position of Finland was complex. In World War II, Finland, after having signed the Moscow Peace Treaty in 1940, decided nevertheless to attack the Soviet Union as an ally of Nazi Germany in 1941, in what is known in Finland as the Continuation War. At the end of the war, Finland still remained in control of most of its territory, notwithstanding their status as losing powers. Finland also had a market economy, traded on the Western markets and joined the Western currency system. Nevertheless, although Finland was considered neutral, the Finno-Soviet Treaty of 1948 significantly limited the freedom of operation in Finnish foreign policy. It required Finland to defend the Soviet Union from attacks through its territory, which in practice prevented Finland from joining NATO and effectively gave the Soviet Union a veto in Finnish foreign policy. Thus, the Soviet Union could exercise "imperial" hegemonic power even towards a neutral state. The Paasikivi–Kekkonen doctrine sought to maintain friendly relations with the Soviet Union and extensive bilateral trade developed. In the West, this led to fears of the spread of "Finlandization", where Western allies would no longer reliably support the United States and NATO.