THE ISSUE: How should America use its influence in a world where being a superpower doesn't get you what it once did? As instability and human tragedy in Iraq, Afghanistan, Libya and Syria have shown, the U.S. alone cannot impose solutions or force the surrender of adversaries like the Islamic State group, which cannot be deterred by the threat of nuclear attack.
WHERE THEY STAND
Donald Trump says his approach is defined by the phrase "America First." He says, for example, that if allies in Europe and Asia won't pay the full cost of U.S. contributions to their defense, then the U.S. should let them defend themselves. He is sour on "international unions that tie us up and bring America down."
Hillary Clinton takes the view that America benefits from a wide network of alliances, both for security and for economic strength. She says she would work to widen and strengthen that network. She criticizes a "go-it-alone" approach for the U.S. and asserts that international partnerships are "a unique source of America's strength."
WHY IT MATTERS
The way America wields its power around the world affects people in every walk of life, in every corner of the country. Going to war in Afghanistan in 2001 and Iraq in 2003 profoundly changed the lives of tens of thousands of people whose loved ones were killed or grievously wounded. It also raised questions that confront Clinton and Trump: How can American influence be used most effectively to protect the homeland and prevent future wars?
In Iraq and Syria, President Barack Obama has chosen not to use the full force of the U.S. military against IS. Instead he has sent small numbers of troops to prod and coach local forces to do the main fighting, backed by U.S. airpower. He says this is more likely to create a durable success than fighting the Iraqis' and Syrians' wars for them. Trump says this is an abdication of a commander-in-chief's responsibility to extinguish as quickly as possible the most immediate threat to the United States. Clinton supports the thrust of Obama's approach to avoiding another U.S. war in the Mideast.
The Iran nuclear deal, which Trump trashes and Clinton praises, is an example of diplomacy with the potential to change the course of history, for better or worse. Critics like Trump say it opens the door for Iran to get its hands eventually on nuclear weapons, which would threaten America. Clinton says it blocks that path and provides possibilities for change in Iran that could reduce the chance of war.
At its core, the discussion about U.S. leadership gets down to this: How much can the U.S. accomplish acting alone, compared with allying itself with like-minded nations? The question applies not just on the military front but also in economics. Trump argues the U.S. gets too little out of current trade arrangements as well as decades-old security partnerships like NATO, which is anchored in Europe but traditionally led by the U.S. He has called NATO "obsolete" and a bad deal for America.
Clinton, by contrast, sees NATO and alliances with Japan and South Korea as a pillar of U.S. strategy for promoting peace and preventing war.
Trump is right when he says NATO was created to confront a threat - the Soviet Union - that no longer exists. The question is whether the alliance is capable of adapting to 21st century threats like a resurgent Russia, instability in the Middle East and the appeal of the Islamic State group. Whereas Trump suggests the U.S. can be better off going it alone, Clinton aligns herself with the more traditional notion that there is strength in numbers.
This story is part of AP's "Why It Matters" series, which will examine three dozen issues at stake in the presidential election between now and Election Day. You can find them at: http://apnews.com/tag/WhyItMatters.