The GOP presidential candidates are discussing foreign policy. Finally! Over the next half decade, the occupant of the Oval Office may well confront foreign challenges far more serious than those arising on the domestic front.

It’s important for Americans to know whether these candidates understand the challenges we face in the world. America is truly at a crossroads. One path — the one we are on — leads to the decline of American power, influence and respect. The other path leads to the restoration of American power and leadership along the lines established by Ronald Reagan — namely, in seeking peace through strength.

How the GOP candidates respond is all the more important because many in Washington seem to think President Obama’s foreign policies have been his greatest success. They mistakenly think killing Osama bin Laden and other high-profile terrorists is tantamount to winning the war against terrorists. They cheer Mr. Obama’s popularity overseas as proof of his success even though some of it reflects delight in his weakening of American power.

The dizzying fog of Mr. Obama’s many presidential summits and diplomatic initiatives can make micro-successes look like great victories to some. But a cleareyed view sees U.S. power and influence waning at the same time the serious challenges and threats are growing.

Nowhere is this more evident than in American military power. In early 2009, the administration canceled or delayed more than 50 major weapons programs. The Obama administration already has cut roughly $450 billion from the current and future defense budgets, and the recently enacted Budget Control Act reduces the fiscal 2013 budget authority for defense about 15 percent below fiscal 2011 levels. Moreover, if the deficit supercommittee fails to agree on a deficit-reduction plan, it will trigger $1.2 trillion in automatic cuts, half of which could fall on national security. That would devastate America’s military capability.

The GOP candidates for commander in chief will have a chance to explain what they would do about this at Tuesday’s debate in Washington, D.C. Co-sponsored by CNN, the Heritage Foundation and the American Enterprise Institute, that debate will focus exclusively on defense and foreign-policy issues. It will give the candidates the opportunity to get specific not only about new resources, but about how we are to dig ourselves out of the hole we are in on national defense.

The candidates also need to show they understand the complex array of strategic threats and challenges America faces. Do they understand, for example, that the struggle in the Middle East is not only about the potential threats of Islamist extremism and Iran’s nuclear program, but about Iran asserting dominance over Sunni Arab nations? That Russia is trying to re-establish its sphere of influence along its borders, using strategic arms talks with America to shield them from criticism?

Do they have a plan to pick up the pieces after most U.S. military forces are pulled from Afghanistan, possibly leaving us once again to deal with Taliban and extremist control over parts of that country? What would they do about Pakistan? Should we continue giving aid to a country that clearly aids and abets terrorists threatening our country?

Finally, do they have a plan for dealing with China? Beijing intends to push back and even supplant American power in East Asia, all the while using trade and financial ties with us as a buffer to shield it from tougher U.S. action. Would the candidates, for example, reverse Mr. Obama’s decision not to sell Taiwan any F-16 C/Ds to replace the almost 40-year-old F-5s — a decision that has failed to appease the Chinese as the administration had hoped?

These are but a few of the challenges facing the next president. The country may be in an economic crisis, but a national security one is looming around the corner unless the next president changes course.

Kim R. Holmes, a former assistant secretary of state, is a vice president at the Heritage Foundation.