By Bill Emmott, independent writer, lecturer and international affairs consultant

The comprehensive victory by Donald Trump and his Republican Party in the U.S. election on Nov. 5 undoubtedly signals a big change in the role that America will play in the world, at least for the next four years but potentially for many more. It no longer can be said that Trump's presidency in 2017-21 was an aberration against the normal trend of American global leadership. For the moment, we must conclude that President Joe Biden's partial restoration of the familiar form of American leadership was the aberration and that Trump is defining a new normal.

Yet to acknowledge this does not, unfortunately, provide an immediate means of understanding what the changed definition of American leadership will look like, nor of where it might leave the U.S. or the world at the time of the 2028 presidential elections or beyond. For the purposes of this column, let us put to one side what Trump and his followers may do to America's institutions of government or to its judicial system, important though that will be domestically; this will affect America's reputation as a democracy and liberal society, but will not directly affect foreign policy except in the unforecastable eventuality that it results in internal strife.

We do know for sure what the attitude of the new Trump administration in foreign affairs will be: it will be confrontational with allies, transactional with everyone, and guided by the principle of "America First," if that slogan can really be called a principle. In many respects, the administration will also be unpredictable, as it is well known that President-elect Trump is a man who often changes his mind, even quite abruptly. But beyond that, little is certain.

Two reasons lie behind this uncertainty. One is simply the difference between campaigning and governing. To campaign is to persuade and to seek popularity; to govern, as the old saying goes, is to choose. A key characteristic of Trump's style of campaigning is his willingness to say anything that he thinks may go down well with voters or keep him in the spotlight, regardless of any major contradictions in his promises. When governing, choices cannot be avoided.

The second reason is that America remains the world's largest economy but is also a country with global economic and security interests and exposures. This conceptual contradiction makes "America First" a much harder idea to implement than it might have sounded on the campaign trail. While many people fear an American reversion to the sort of isolationism and detachment that was seen during the 1920s and 1930s, the reality of America's global security and business interests makes this a remote prospect.

Look just at Elon Musk, the billionaire who backed Trump's campaign most conspicuously and since the election has stuck close to his side: Musk's electric vehicle business, Tesla, builds cars and components in factories in Germany, the Netherlands, Canada and China as well as the United States; his Starlink satellite-based internet business is global, and his SpaceX business has customers worldwide too; all depend on global supply chains for their manufacturing. "America First" means little to him, and could even pose a threat to his businesses.

The campaigning and the promises are therefore riven with contradictions. Most relevant to foreign affairs is the contradiction between Trump's loud demands that America's allies in NATO in Europe and in bilateral security alliances in Asia should contribute more in terms of defence spending and military capabilities, while declaring, also loudly, that he plans to impose high tariffs on American imports from Japan, Europe and elsewhere. This would make it harder for those allies to contribute more and would raise America's own defence procurement costs since it depends on Europe and others for many key supplies.

Another contradiction, which is related to that threatened trade war on America's own allies, is that Trump campaigned hard on the idea that he would seek "peace through strength," by building up America's own defence spending and by confronting China in every way necessary, and yet his plans threaten to reduce that very strength by eroding America's alliances in the Indo-Pacific.

The American defence industry does not have sufficient production capacity to supply current U.S. military needs, let alone a defence expansion or a conflict with China: it depends crucially on co-production with allies and partners, notably including Japan and South Korea. America's China strategy, even in the minds of Republican Party strategists, has long depended on persuading countries in the region to at least stay neutral or ideally to lean toward America: slapping high tariffs on goods from India, Vietnam, the Philippines and other "strategic partners", in the preferred diplomatic jargon, is hardly the best way to seduce them.

Much therefore depends on how these contradictions are resolved and what America First truly comes to mean. An immediate question concerns his campaign pledge to "end" Russia's war in Ukraine by negotiating a peace: if he makes a serious attempt at doing that, he will also have to take account of Russia's strategic partnership with China and of Russia's use in the conflict of soldiers and munitions from North Korea. Beyond that important question, however, he and his national security team will have to work out how they can stand up to China while simultaneously engaging in trade wars.

In the case of Japan, there may be some room for compromise given that even the most zealous of America Firsters cannot deny the importance of U.S. military bases in Japan and Japan's own defence build-up to the anti-China strategy. But for many countries that lack such close ties to the U.S., moves in recent years to join China-led groupings such as the BRICS will look to have been a prudent hedging of bets.

There is no room to doubt that the Trump administration will be at least as tough in its competition with China as the Biden team has been. The contradictions concern the potential effectiveness of that policy, not its direction. The Trump administration will demand more for Taiwan in return for its support and protection, but will not knowingly seek to open the doors to a Chinese takeover. Given Trump's past dialogue with North Korea's Kim Jong-Un, we can expect him again to try to open up personal talks with Kim, though the North Korean troops fighting alongside Russia will likely delay any such initiative.

The reelection of Donald Trump spells the end, for now, of the old form of American leadership. With the world's largest economy and military force, and with interests all around the globe, America will still, however, remain a leader. What we have yet to discover is where that new form of leadership is going to take us and how sustainable it will be.