Has liberalism or realism dominated American foreign policy since World War II? What are some examples of each?

There are examples of both in the period after World War II, but I will say that realism has largely dictated American foreign policy after World War II.  The United States invaded Iraq in the Gulf War in 1990 in order to defend Kuwait, an ally, but the United States had significant oil interests in the region and wanted to make sure that Saddam Hussein did not gain too much power.  Thirteen years later, the United States saw an opportunity to change the leadership in Iraq and remove a potential threat in the region.  Claiming world security, the United States invaded once again in order to secure vital economic interests in the region.  Another example of realism is the example of American intervention in the developing world in the Cold War.  Part of the containment strategy was to prevent the spread of communism.  This also allowed the United States to show strength to the Soviet Union in an attempt to intimidate the Soviets.  The United States fought to ensure regimes in Iran, Guatemala, and South Vietnam remained in power because those regimes were favorable to the United States.  The people who lived in those areas hated the brutal and corrupt regimes who ruled these countries and sought to overthrow them.  The United States, seeking to show its ability to back an ally, sent money, supplies, troops, or sometimes all three of these to defend these regimes.  This led to a series of coups in Guatemala that killed thousands, the Vietnam War, and a legacy in Iran that haunts the United States to this day.  Finally, in Afghanistan, the United States helped the mujahideen in their efforts to resist Soviet invasion.  The Afghan fighters received arms and American advice and turned the Soviet-Afghan War into a very costly one for the Soviets.  After the war, the United States left the region, and Afghanistan quickly deteriorated into a state prone to terrorists and disorder.  
There are some examples of liberalism on the part of the United States.  The United States used the Marshall Plan in Europe in order to rebuild the area.  This established vital American trading partners and warded off communism.  The United States also joined NATO and became the most powerful nation in the alliance—even though being part of it was not necessarily in the United States' best interests.  The idea behind the Marshall Plan and NATO was to ensure economic and military cooperation and hopefully limit war through diplomacy.  The United States also signed the North American Free Trade Agreement (NAFTA) with Canada and Mexico in order to achieve greater economic gains for the region.  
While the United States' actions exemplify both realism and liberalism, I find that realism dominates more than liberalism.  The United States claims humanitarian aims in its military interventions, but there is often an underlying economic or geopolitical reason for acting in that particular country.