As President Trump grouses about having to visit Canada and looks for new ways to punish the United States’ neighbor to the north, it is instructive to recall an earlier presidential trip.

President Ronald Reagan traveled to Ottawa in March 1981, shortly after taking office. The two nations were at odds over acid rain, fishing, automobile trade, a gas pipeline, Central America policy and more.

But in an address to Parliament, Reagan said, “A final word to the people of Canada: We’re happy to be your neighbor. We want to remain your friend. We’re determined to be your partner.”

The abject willingness of Republican politicians to discard their supposed fidelity to Reaganite principles has become so familiar in the Trump era that we hardly notice any longer. But a visit to the Ronald Reagan Presidential Library and Museum here nonetheless is a shocking reminder of the vast gulf between the two Republican presidents — not only on issues but also in philosophy, outlook and character.

Reagan’s expression of good neighborliness is one of the first exhibits you encounter in the museum. But earlier in the timeline you come across this, about the release of U.S. hostages held by Iran for 444 days: “President Reagan sends former President Jimmy Carter to meet the hostages in Germany and escort them home to freedom.”

Can anyone imagine that kind of gracious gesture from Trump toward his predecessor? ...