IT IS impossible to know who American voters will choose as their next president. But it is certain that the choice will have profound consequences for the entire world.
It is difficult to know what role foreign policy will play in determining who will next occupy the Oval Office. The 2016 election is still 17 months away. A lot can happen between now and then.
The Democratic and Republican parties' nominating contests will play out over the next year. Former Secretary of State Hillary Clinton is the Democratic front runner, though her nomination is not a foregone conclusion. In any case, foreign policy probably will play little role in the decision as the issues that most concern voters likely to participate in the Democratic primary elections are domestic and economic in nature.
The Republican side is far more crowded and uncertain, and it seems far likelier that foreign policy will play a large role in choosing the party's nominee. The economy is improving under President Barack Obama, making it a less attractive political target. Global turbulence, by contrast, has given the Republicans more room to attack Mr Obama and the Democrats.
Nonetheless, a few foreign policy issues will dominate the conversation in both parties. One is trade. Mr Obama is seeking Trade Promotion Authority, a necessary prelude to gaining congressional support for the Trans-Pacific Partnership (TPP), which would reduce barriers between the US and 11 other Pacific Rim countries. Many - but not all - of the Republican candidates back the TPP; the politics on the Democratic side is more hostile to the deal, making it potentially risky for any Democratic candidate to support it.
A second issue certain to dominate both parties' nominating debates is Iran and the international negotiations to contain its nuclear programme. One can expect many of the Republican candidates to be critical of any proposed deal. Democratic candidates are more likely to be sympathetic to whatever is negotiated; but there are certain to be differences among candidates on both sides.
A third issue is climate change. Pope Francis is expected to boost the issue's salience in a major statement on it today. Likewise, planning for the United Nations Climate Change Conference in Paris in December will keep the issue in the news. Democrats will be more supportive of more far-reaching US commitments although, again, differences of view will emerge on both sides.
A fourth cluster of issues involves the Middle East. There is little appetite on either side for large-scale military intervention in Iraq and Syria to counter the Islamic State of Iraq and Syria. But there will be heated debate - and considerable posturing - over what should and should not be done.
Then there are all the other issues, from Chinese assertiveness in Asia to Russian revanchism in Ukraine. The rhetoric, especially on the Republican side, will be muscular.
One hopes that what emerges from the parties' nominating processes is insight into how the successful candidates answer three big questions.
The first concerns how much importance the nominee assigns to foreign policy. If one thinks of national security as two sides of a coin, with foreign policy on one side and domestic policy on the other, how likely is each to land face up for the next president? This is the classic "guns versus butter" debate over how resources, from dollars to presidential attention, should be allocated.
Second, what are the purposes and priorities of foreign policy? The realist tradition in international relations focuses on influencing other countries' foreign policies and places less emphasis on their internal affairs. The main alternative tradition takes the opposite tack, arguing that other countries' domestic affairs are what matter most because it is believed that how a government behaves at home affects how it acts abroad.
According to this idealist view, countries that are democratic and treat their citizens with respect are more likely to treat other countries' citizens with respect. The problem, of course, is that affecting the trajectories of other societies is typically a difficult, long-term proposition. In the meantime, there are pressing global challenges that need to be addressed, sometimes with the assistance of unsavoury regimes.
The last question concerns the nominees' approach to the execution of foreign policy. What will be their preferred mix of unilateralism and multilateralism, and which tools - from diplomacy and sanctions to intelligence operations and military force - will they reach for most often?
The answers to these questions should become clearer during the campaign and Americans will gain a better sense of who to vote for and develop a better sense of what to expect in January 2017, when the 45th US President takes the oath of office.
The writer is president of the Council on Foreign Relations and the author, most recently, of Foreign Policy Begins At Home: The Case For Putting America's House In Order.