A country, which had the small government and a rather pacifist foreign policy as the main principle of the governing between 1776-1941(there have been small advantures in 1898 and 1917-19, but still, they are really nothing if you consider the post Truman Doctrine Period), suddenly felt the urge of controlling the globe and containing the communism and eventually to destroy it. It seems weird to me, even when Nazi Germany invaded France in 1940, most Americans were skeptical about going to another war, and US Military was kept small, and in 1950, when Chinese-Soviet backed North Korea invaded the South, a country which majority of the US Public of 1940s never heard of, they intervened very quickly.
I've few questions about it actually, well I should say that I'm a big despiser of conspiracy theories, were there anything going on behind the closed doors? Did a business&politics elite, that aren't very publicized(yes those mysterious dark powerful old men), put pressure on Truman Administration when they saw that European countries like UK and France are no longer in position to be a world police after 1945, and save their global interests from the communist expansion?
Another one, when did the US Government know the Cold War was incoming? Were they already preparing for it before the end of World War II? Did the consecutive land gains of Red Army in late 1944 started to alarm them or were they still happy about Soviets destroying Nazis?
Also, did they actually believe that Soviets would take over the world? Or was it an exaggeration to keep the military industrial complex alive, while the Soviets didn't pose a direct threat to the US Soil. Or did they?
And why did Cold War started to calm down after mid-60s? While in the early 50s, it was the main agenda of the US, however, in 80s, the main public almost forgot that there was still a Cold War. Reagan told a few humorous quotes about Soviets in public speechrs, few coups in Latin America and Middle East, and that was about it. The anticommunist soul was dead.
First, the US fought the War of 1812, the Mexican-American War, the Indian Wars, the Spanish-American War, and World War 1 between the end of the Revolution and World War 2. The US also had forces in Russia during their Civil War, as well as sending troops to Nicaragua, Haiti, and the Dominican Republic, as well as fomenting rebellion in Colombia to create Panama. The US also forced Japan open militarily, fought a few battles (Pyongyang, Gangwha Island) with Korea, and participated in the Boxer Rebellion in China. The Marine Corps was founded to fight pirates (who had their own country/sponsors) in North Africa. Teddy Roosevelt sent the Great White Fleet around the world, and he implicitly gave Japan permission to take over Asia. The United States was far from pacifist or non-interventionist in foreign policy.
Second, the dynamic between the US and the USSR is a complicated one. After Russia left WW1, they had a civil war in which US troops fought FOR the Tsar, who lost to the Communists. In WW2, the Soviets took the brunt of Germany's might and lost way more people than pretty much everyone else combined. Stalin wanted D Day and a second front opened much earlier than it was and he felt that the Allies were dragging their feet, letting Soviet soldiers handle the Germans and get killed. So when the War ended, and the Soviets decided to set up buffer states and Stalin started pushing Comintern, he didn't really care what the Allies thought. Churchill urged Roosevelt to continue the War, to use atomic weapons and go after Stalin; he knew what a terrible guy he was (he coined the phrase "iron curtain". With the distrust between Churchill and Stalin, and Stalin's memory that the US had supported the Tsar AND dragged it's feet on opening the second front, there wasn't much hope to remain buddy buddy after the war.
Did the US see the cold war coming? Yes and no. From troubles with the War crimes trials at Nuremberg, to arguments about Berlin, the cold war began very quickly, pretty much as soon as the War was over. Also, Truman was pretty much in the dark until Roosevelt died and he took over. While he wasn't an idiot, he was playing catch-up which gave the Soviets a bit of an edge.
Did the US believe communism would take over the world? Yes. See the Domino Theory on this. The Truman doctrine was to help all countries in their struggle against communism, lest one fall and then they all fall. Going to Vietnam was based on the Domino theory. People were genuinely scared that communism would win.
The cold war died down in the 60s a bit because Stalin died and Kruschev backed away from the craziness that was Stalin. That said, the 60s still had the Cuban Missile Crisis and the Space Race. Once Khrushchev was ousted and replaced by Brezhnev, the USSR became a bit more aggressive, but the 70s brought detente and missile reduction talks.
The 80s, however were far from calm. The cold war was very real in the 80s. Reagan was percieved as a zealot, as someone who WOULD start a nuclear war. From pushing SDI (Star Wars), to exercises like Able Archer 83 which simulated an attack on the USSR, Reagan was seen as a real threat, and there were those who pushed for a preemptive Soviet strike. In 1983, with Able Archer and the shoot down of a Korean Air flight carrying a member of Congress, the world had not been closer to nuclear war since the Cuban Missile Crisis. The 80s were a terrifying time.