The Trump administration’s attack on Canada and Mexico is cracking the foundation of the United States’ global power. Friendly relations with neighbors enhance security, promote mutual economic gain, and make it easier to handle transnational challenges. Weak neighbors export trouble. Today, Canadians perceive U.S. President Donald Trump’s high tariffs and belittling of their country as the “51st state” as assaults on Canada’s economy and independence. In Mexico, Trump’s economic coercion and threats of military intervention have revived deep historical anxieties about gringo hegemony.

The North American partnership took generations to forge. During the nineteenth century, the United States’ foreign policy concentrated on its home continent, where it competed with France, Mexico, Russia, Spain, the United Kingdom, and indigenous peoples for dominance. As the United States became a global power in the twentieth century, U.S. strategists took North American relations for granted. Washington turned its attention to Europe, the Asia-Pacific, and the Middle East. Since the closing years of the Cold War, however, U.S. presidents have recognized the need to invest in North America. Ronald Reagan laid out the strategic rationale clearly when he launched his presidential campaign in 1979: “The key to our own future security may lie in both Canada and Mexico becoming much stronger than they are today,” he explained. “It is time we stopped treating our nearest neighbors as foreigners.”

Geopolitics begin with geography, and the United States’ geography gives it an enormous advantage over the world’s other major powers. China and Russia each have to manage relationships with 14 neighbors, many of which are unfriendly or carry resentments from histories of conflict. India must deal with six, including a nuclear-armed rival. All three powers’ access to the world’s seas depends on passage through maritime chokepoints. In contrast, the United States borders two benign neighbors and enjoys open sailing on two vast oceans. Close cooperation with Canada and Mexico enhances homeland security, attracts investment, promotes innovation, creates a huge market rich in resources and talent, and helps all three countries deal with criminal networks, illegal immigration, and threats to clean air, water, and wildlife.

A safe, resilient, and adaptive North America is the starting point of U.S. global strategy. Washington is best positioned to navigate an uncertain world when the United States, Canada, and Mexico present a common front. Yet Trump’s bullying is driving the United States’ natural partners away. The president is destroying a precious geopolitical asset: a strong continental base from which Washington projects power.

William Seward, who served as U.S. secretary of state from 1861 to 1869, first appreciated that good relations with the United States’ neighbors would boost the country’s global influence. He asserted that to have “control of this continent is to be . . . the controlling interest in the world.” When he was negotiating the U.S. purchase of Alaska in 1867, Seward observed that Alaska and the Aleutian Islands “extend[ed] a friendly hand to Asia.” He also tried to buy Greenland, Iceland, the Hawaiian Islands, and islands in the Caribbean, hoping that the purchases would help secure the United States’ oceanic approaches.

Seward recognized, however, that the United States could also boost its power by strengthening economic links, not just by adding territory. He abhorred European imperial conquests of foreign lands. He had also opposed the 1846–48 U.S. war with Mexico. At the end of the U.S. Civil War in 1865, Seward prevented Generals Ulysses Grant and Philip Sheridan from invading Mexico to depose the Habsburg archduke who the French had installed as emperor. He reasoned that an invasion would arouse Mexican hostility and that the Mexicans could depose the interloper themselves—which they did, in 1867.

The United States’ geography gives it an enormous advantage.

Seward dealt diplomatically with the United States’ northern neighbor, too. In 1867, the British government united its North American colonies to form the Canadian Confederation, in part to deter Washington from invading Canada in retaliation........

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