How has the United States' role in the world changed in the years from 1945 to the present and what actions by the United Stets in the international arena have generated the most controversy?

Since 1945, the United States has taken a more active role in world affairs.  Directly after WWII, the United States rebuilt the economy of Western Europe through the Marshall Plan.  The United States also sought to stop the spread of communism by assisting governments which were friendly to its interests, such as the pro-Western governments of South Vietnam and Iran.  Sometimes propping up governments friendly to American interests backfired, leading to popular unrest that led to the overthrow of the South Vietnamese and Iranian governments by the people who lived there.  
America has also assisted in humanitarian causes throughout the world, such as leading the way to hunt down Nazi war criminals and helping to put down genocide in Bosnia.  Once again, America's humanitarian record has been uneven—America did little to end the genocide in Rwanda and the aid it provided in Somalia has proven to be quite meager.  Also, America's role in remedying the current refugee crisis in the Middle East is quite small when compared with the role of other Western European nations.  America has also ignored the human rights violations in China and Saudi Arabia, leading to charges of hypocrisy.  
America has also assisted oppressed groups as they have fought against invaders and dictators.  America toppled Saddam Hussein's government and it assisted fighters in Afghanistan in resisting the Soviet Union's invasion.  These have had unintended consequences—Iraq has not been stable in well over a decade and America had to invade Afghanistan after the attacks of September 11 in order to persecute Osama bin-Laden and Al-Qaeda.  
American foreign policymakers have taken action to safeguard the nation's economic and security interests; however, sometimes its actions have proven to lack foresight and have not acknowledged the will of the people who live in those regions.