Various treaties and acts were put in place during the period before World War Two. These treaties, though they may seem harmless, also each contributed to the start of the war in their own ways.
Treaty of Versailles (1919)
This treaty was made to ensure a lasting peace right after World War I through trying to punish Germany and setting a League of Nations to help solve diplomatic problems. The treaty did just the opposite causing bitterness from Germany and helping to spark World War II. The Treaty of Versailles put a lot of restrictions upon Germany. Through it they were given restricted military, forced to pay for the wartime (33 million dollars), their colonies were taken away, and on top of it all, they were blamed completely for the war. This, of course, caused a lot of bitterness in Germany which explains the tensions tension that grew and the rise of Hitler. By placing so many restrictions on Germany, the people grew to nationalism and the hopes for a some savior would arise. The bitterness caused Hitler to reject the treaty, and because the other countries were so afraid of war, they did nothing.
The Reciprocal Trade Agreements Act (1934)
An act that allowed the president to establish tariff-reduction agreements with foreign countries without the approval from congress. Led to the General Agreements on Tariffs and Trade in 1947. This idea brought America into greater involvement with Europe which would bring the country closer to World War II.
The Johnson Debt Default Act (1934)
An act created prohibiting defaulting governments from further borrowing in American markets for several crucial years. It was created to encourage European countries that had defaulted to pay back debts from World War One. Rather, the act caused the countries to simply stop paying which worsened the depression and increased nationalism in Germany and Italy.
The Neutrality Acts of 1935, 1936 and 1937
The Neutrality Acts stated that the US must place an embargo on arms sales for all countries involved in the war. Because of these acts, the United States wasn't allowed to interfere with any issues involving war. As such, whenever Germany and Japan began showing signs of aggression, the United States did nothing when doing something could have helped prevent the war.
The Nonaggression Pact (1939)
(Also known as Nazi-Soviet Non-Aggression Pact, The German-Soviet Non-Aggression Pact, or the Ribbentrop-Molotov Pact). This was an agreement between Nazi Germany and the Soviet Union stating that they would not attack each other. If there was a problem between the two countries, they would work through it agreeably. It was decided upon by Adolf Hitler who was planning to obtain the land of Poland and did not want the Soviet Union to come to Poland's aid should they go into war. The act was placed mainly to ensure that the Soviet Union wouldn't become immediately involved in the war. If it had, Germany's plan would have failed.
The Neutrality Act (1939)
A very important decision made by President Roosevelt, and voted for by Congress, that eliminated the arms embargo in the case of a war in some place. By doing this, the United States began to become increasingly involved with the war.
The below acts were made during the war that mainly caused the United States to become closer to Europe and its struggles. By involving itself with European affairs, the United States became the inevitable target of the Axis attacks.
The Destroyer Deal (1940)
In September of 1940, Roosevelt made a destroyer deal with Great Britain providing them with 50 overage destroyers in return for a string of bases in the Caribbean.
The Lend-Lease Act (1940)
Britain's reserves of money were horribly low from the war, and thus they could no longer purchase arms from the US. Roosevelt thus created this act which allowed for arms to be "borrowed" by Britain to be returned after the war. This act sparked controversy among political figures, though it was eventually passed. This brought America closer to the war economically.
Atlantic Charter (1941)
Britain was failing miserably even at sea (which was its specialty), and thus Roosevelt responded by expanding the merchant escort zone which meant that US ships would escort merchant ships farther into the ocean. This proved a major aid to Britain, and thus Britain began failing not as badly. Churchill refused to surrender.