President Donald Trump has delivered on at least one promise: He has made “America First” the guiding principle of U.S. foreign policy. At this week’s NATO summit, when he hectored allies over their defense budgets, he followed a pattern established at the June 8 Ottawa meeting of the G-7: alliances are useful only if they benefit America’s direct commercial and financial interests. For Trump, the solidarity of the West is a liberal myth. His warm embrace of North Korean dictator Kim Jong Un in Singapore on June 12, along with his upcoming Helsinki conference with Russian President Vladimir Putin, amplify the point: The old world order is dead, and Trump’s foreign policy will be driven by short-term deals, even with dictators.

It might make some Trump loyalists happy to think that the president is destroying something called “liberal internationalism.” But actually the world order he is attacking has been a core value of the Republican Party ever since Dwight D. Eisenhower turned back the dangerous isolationist faction of the GOP. Every Republican president since Ike has adhered to the basic theory of internationalism: that alliances make America stronger, safer and richer.

Isolationism has a long history in the Republican Party. Following World War I, in which over 100,000 Americans died, millions of citizens asked why the United States should remain entangled in world affairs. With a huge internal market, peaceful borders and two giant oceans on either side, the USA seemed blessed by destiny: Wars that might break out around the world could not trouble the American homeland.

Republican presidents Warren Harding, Calvin Coolidge and Herbert Hoover trimmed their sails to this isolationist sentiment and avoided entangling alliances with European nations. This scorn for the old world aligned with a growing chorus of domestic critics of U.S. foreign policy, which asserted that the Great War had only benefited a small caste of arms manufacturers and bankers. So alleged Senator Gerald P. Nye (R-ND), who chaired a Senate investigation into the munitions industry.

Under Nye’s influence, Congress passed Neutrality Acts in the 1930s to stop the export of arms and loans to any belligerent nations. ...