The League of Nations did not do as well as it could have because the United States did not participate in it.
Constantinople fell to the Ottoman empire in May 1453.
This era would be known as the Cold War period. This period of time is when the US and Soviet Union faced off in a battle of political power and global influence. The US (in support of capitalism) and the Soviet Union (in favor of communism) constantly tried to limit the power of each other. In order to show their political power, each country made allies.
NATO (North Atlantic Treaty Organization) was a political and military alliance between countries like the US, Great Britain, France, Denmark, Italy, and Canada which focused on stopping the spread of communism. These countries were worried that nations influenced by the Soviet Union would turn into one similar to the Soviet regime. These countries did not like this communist regime, as the rights and individual liberties of the citizens were severely limited.
