After the Axis powers were defeated and the victorious democracies began rebuilding an open world order, it took decades for the flow of trade, money and people to regain momentum. Global trade did not recover to its 1914 peak until the 1970s, and capital mobility — the scale and ease of money flows — did not recover until the 1990s. Once these flows gained speed, however, they thundered along right up to the financial crisis.

Today, 2008 looks to be as clear a turning point as 1914. With global demand weak, and many nations erecting import barriers, trade is slumping. Measured as a share of global gross domestic product, trade doubled from 30 percent in 1973 to a high of 60 percent in 2008. But it faltered during the crisis and has since dropped to 55 percent.

The flow of capital — mainly bank loans — is retreating even faster. Frozen by the financial crisis and squeezed afterward by new regulations, capital flows have since slumped to just under 2 percent of G.D.P. from a peak of 16 percent in 2007.

The flow of people is slowing, too. Despite the flood of refugees into Europe, net migration from poor to rich countries decreased to 12 million between 2011 and 2015, down by four million from the previous five years. Between 2009 and 2014, the number of Mexicans leaving the United States outnumbered new arrivals by 140,000, and that was before Mr. Trump’s first anti-Mexican tirades.

In an echo of the 1930s, the slowing of trade, global investment and migration are further weakening the global economy. There are many reasons to expect that this new age of deglobalization will last, as the postwar order is under assault from both popular autocrats in emerging powers like Russia and China, and populist candidates in Western democracies.

The recent trade boom was fueled by relatively simple deals that cut import tariffs. But trade deals have become more complex, and now take much longer to complete. At the same time, the world’s major economies have imposed hundreds of protectionist measures since 2008, led by India, Russia, China and the United States. And once such protectionist walls spring up around one industry, they tend to grow and spread to other industries.

Changes in China’s economy will further slow trade. When China opened up in the 1980s, its vast population turbocharged global trade almost overnight. Nations all over the world prospered by supplying raw materials and parts to plants in China, and later in countries like Poland and Mexico. Before 2008, much of the global trade boom involved intermediate goods traveling within these supply chains. But this trend has reversed. Supply chains are contracting, particularly as China moves to make its economy less dependent on trade, and its factories learn to make more parts at home.