WASHINGTON — In February 1945, in the twilight of World War II, Franklin D. Roosevelt, Joseph Stalin and Winston Churchill convened in Yalta, a Russian resort town in the Crimea, to deliberate on the direction of the war and the peace to follow. They agreed to a postwar order managed by Roosevelt’s “Four Policemen” — the United States, Britain, Russia and China.

Roosevelt was convinced he could cajole Stalin into keeping his Yalta commitments to collective security and an undivided Europe. Stalin had a very different vision: a world shaped by spheres of influence within which the will of the strongest prevails. In the Soviet sphere, darkness descended on Eastern Europe for 45 years.

It fell to President Harry Truman to contain Soviet expansionism. He built America’s first peacetime alliances, starting in Western Europe, then in Asia. The United States took the lead in shaping the norms, rules and institutions of what became the liberal international order, including the United Nations, the international financial institutions and the Marshall Plan.

The liberal order led by the United States favored an open world connected by the free flow of people, goods, ideas and capital, a world grounded in the principles of self-determination and sovereignty for nations and basic rights for their citizens. It did fall short of its ideals, often in Latin America and Southeast Asia. Yet despite the hair-trigger tensions of the Cold War, it produced decades of peace between the great powers while building shared prosperity.

The postwar order that America built now is facing acute challenges, including from old competitors. Russia’s president, Vladimir Putin, is no Stalin and Russia is no Soviet Union. But Mr. Putin does seek to recreate a Russian sphere of influence while picking apart the liberal international order that prevailed in the Cold War. China remains focused on stability at home, but the “new model of great power relations” it has proposed to the United States would have us stick to our side of the Pacific and let China play the pre-eminent role on its side.

America’s allies in Europe and Asia are fixated on whether the Donald Trump administration will reject the re-emergence of spheres of influence or embrace them. They worry that, in his campaign, Mr. Trump seemed to approve of the “strong” leadership of autocrats and favor a transactional approach to Mr. Putin. He showed little concern about Russia’s cybermeddling in our election or aggression in Ukraine while suggesting that NATO is “obsolete.” He argued that the United States should get out of the business of “defending the world” and described Japan and South Korea as free riders that should pick up the burden of their own defense and nuclear deterrence.

He has promised to jettison the Trans-Pacific Partnership trade agreement, ceding to China economic leadership and strategic influence in Asia. For many Europeans and Asians, these proclamations translate into a world in which the United States retreats into its cocoon, and Russia and China dominate them in both political and economic spheres.

The United States must not see China or Russia through a zero-sum prism. The Obama administration has deepened areas of cooperation with Beijing, from the Paris climate agreement, the handling of the Ebola epidemic, the Iran nuclear deal and North Korea to joint projects in developing countries. It negotiated the New Start nuclear arms reduction treaty with Moscow and championed Russia’s admission into the World Trade Organization.

Yet when Russia or China challenge the principles of the liberal international order, the United States must stand up to them. In Ukraine, Mr. Putin has sought to change the borders of Russia’s neighbor by force while denying its people the right to decide with which countries, unions or alliances they associate. It is why American support for Ukraine matters.

So does our resolute support for international law in the South China Sea. There, China’s conduct in claiming vast territorial waters and building military outposts on artificial islands risks undermining the freedom of navigation and free flow of commerce upon which our prosperity depends, the peaceful resolution of disputes that undergirds stability and the rights of allies we have vowed to defend.

A sphere-of-influence world would not be peaceful or stable; the United States will not be immune to its violent disruptions. Hegemons are rarely content with what they’ve got; the demand to expand their zones as well as cycles of rebellion and repression within them will lead to conflicts that draw us in. The United States would have to accept permanent commercial disadvantage as economic spheres of influence shut us out or incite a race to the bottom for workers, the environment, intellectual property and transparency.

America’s greatest contribution to peace and progress has been laying the foundation for an open, rules-based, connected world. Now we have to decide whether to continue to defend, amend and build upon that foundation or become complicit in dismantling it.

New York Times