In the XXth century, the US was involved in the two world wars and, after the wars the US became the first economic power in the world.
In 1917 US entered in the first world war with France, England and Italy (to help them to win the war) against Germany.
After the war, US was in a serious economic crisis called Great Depressions. But the President Roosvelt
fight this crisis with a special economic program called New Deal.
When the second world war broke out the US was neutral. The States interested were Germany and Italy against France and England. In 1941 the Japanese attacked Pearl Harbor, so the US entered in the world war. This war ended in 1945 with the destruction of the Japanese town of Hiroshima e Nagasaki
with atomic bombs. The two atomic bombs' names were “Fat Man” and “Little Boy”.
The United Nation is a special institution who was founded in 1845 to helping the countries of the world to solve their problems.
After the world wars, in the second half of the XXth century, the US was involved in many conflicts. For example, the civil war in Vietnam. In fact the US helped the South Vietnamese against the North Vietnamese but many people protested, so the US stopped the intervention.
In 1990 the US was involved in another war, after the Kuwait's invasion, with the United Nation against the Iraq. Then, Kuwait was liberated.
In 2001 US was the victim of a terrorist attack and a lot of people died. This attack was in Washington DC and in New York (the most important cities of US) by Bin Laden. Then, to capture Bin Laden, US invaded Afghanistan and the conflict is still continuing today.