As world leaders wonder whether Mr Donald Trump will launch trade wars on China and Mexico, undermine Nato, and aggressively antagonise much of the Muslim world, it has become increasingly clear that the poor state of the US economy should concern outsiders almost as much as the new President's ill-defined foreign policy plans. The US is still home to the world's largest consumer market, and Mr Donald Trump is likely to stimulate growth. But that won't solve the US economy's underlying issues, and that's a problem for the entire global economy.

The headlines tell a misleading story. US stock markets continue to scale record heights. Mr Trump's speech before Congress at the end of last month sent stocks soaring even higher on hopes that a plan to sharply reduce corporate taxes and spend US$1 trillion (S$1.4 trillion) to upgrade US infrastructure will jump-start US growth. Billionaire investor Warren Buffett recently predicted more boom to come, and backed his opinion with new investment. He's right that US equities have room to run, but that won't help middle-class voters who have lost their purchasing power, and in many cases their jobs, in recent years - the voters who cheer Mr Trump's pledge to "make America great again".

The loss of manufacturing jobs over the past generation has taken a particularly heavy toll on the United States. As Professor Nicholas Eberstadt pointed out in a powerful recent article for Commentary magazine, "per capita growth in America has averaged less than 1 per cent a year" from 2000 to 2016, a sharp decline from the 2.3 per cent annual growth the US enjoyed from 1948 to 2000.

In addition, Mr Trump is right that the official US unemployment rate is deceptive because it does not include the growing numbers of working-age Americans who have stopped actively looking for work. Not coincidentally, addiction to drugs, both legal and illegal, has spiked over the past generation. Prof Eberstadt cites a study conducted in 2016 by Professor Alan Krueger, former chairman of the President's Council of Economic Advisers, which found that nearly half of all male working-age Americans who have dropped out of the economy now take pain medication daily. Among these legions of the disaffected there is demand for system-upending change. Mr Trump has promised to deliver it.

Yet, debt-wary lawmakers will ensure that Mr Trump's promises to invest historic sums to upgrade substandard US roads, bridges, ports, and airports will take longer than he expects and produce a smaller package than promised. Further, the new President won't deliver on the most ambitious of his plans to sharply cut the corporate tax rate. In fact, Congress won't accomplish much of anything until Mr Trump and the Republican Party leadership find a way to credibly end the Obama healthcare programme without stripping millions of voters of their health insurance, assuming that's possible.

Mr Trump will spend much more on the Pentagon, though the new money won't change his opinion that US military might should be used exclusively to advance narrowly defined US security and commercial interests. Trade won't be back on the agenda anytime soon. Mega deals with Asian and European partners are dead. Potential deals with Britain and Japan will take years to negotiate. Other agreements are unlikely, at least for now.

It's also inevitable that, in the US as well as elsewhere, more jobs will be lost to advances in automation and the expansion of artificial intelligence at the workplace. A 2015 study conducted by Ball State University found that automation and related factors, not trade, accounted for 88 per cent of lost manufacturing jobs. In addition, artificial intelligence is reducing the number of - and changing the skill sets needed for - a fast-growing number of service sector jobs. More than half of jobs in the retail sector could be lost, and two-thirds of jobs in the finance and insurance sectors are likely to disappear when computers are able to understand speech as well as humans do, according to the study. That might be the biggest obstacle to Mr Trump's plans to put middle- and working-class voters back to work.

Economic frustration will make the US an even less predictable international actor, because Mr Trump has persuaded his core supporters that outsiders - China and Mexico, in particular - are to blame for "stealing" US jobs and that some allies are free-riding on US military support. If the real US economy continues to generate growth and wealth without jobs, and more working-class Americans feel left behind, Mr Trump's search for scapegoats will become an even more important part of his foreign and trade policies.

As if Mr Donald J. Trump weren't unpredictable enough.

• The writer is the president of Eurasia Group and author of Superpower: Three Choices For America's Role In The World.