When either of the trio, Clinton, Obama and McCain, takes office in January 2009, the wars in Iraq and Afghanistan, terrorism and the nuclear issues of Iran and the Korean Peninsula will be weighing heavy in the new president's in-tray.
Adding to that is the all-time low image of the United States around the world.
McCain has been touting his years of foreign policy experience as a reason to choose him over Clinton or Obama in November.
He voted for military force in Iraq and supported President George W. Bush when he vetoed the war spending bill that would have withdrawn most US troops by March 2008.
McCain said he expects a smaller but long-term US presence in Iraq similar to those in South Korea or Kuwait.
Republican presidential candidate John McCain speaks at a town hall meeting in Rocky River, Ohio February 25, 2008. (Xinhua/Reuters Photo)
On the surface, the Democratic candidates are not that far apart on foreign policy.
Although Obama opposed the invasion of Iraq before joining the Senate and Clinton voted for it, both now favor a phased withdrawal of US forces and the need for Iraqi forces to take over security.
Both Clinton and Obama would not rule out the use of force, even unilaterally, to protect US national security interests.