“Israeli settlements in the West Bank do not violate international law,” according to Eugene Kontorovich

For decades, Israel’s detractors have appealed to consensus, asserting that settlements are illegal because the entire international community agrees they are illegal. As with Jerusalem and the Golan Heights, the Trump administration has refused to be cowed by a hollow consensus. By dissenting, the U.S. has destroyed both the consensus and the frail arguments that relied on it.

The four-page 1978 memo, written by legal adviser Herbert Hansell, was hardly a thorough study. It painted with broad strokes across several issues and cited no precedent for its key conclusions. Most important, its legal analysis of occupation and settlements has never been applied, by the U.S. or anyone else, to any other comparable situation.

Hansell’s memo took two analytic steps. First, it concluded that Israel was an “occupying power” in the West Bank. Next, it invoked an obscure provision of the Fourth Geneva Convention, which says the “Occupying Power shall not deport or transfer parts of its civilian population into the territory it occupies.” Hansell concluded that Jews who have moved past the Green Line into disputed territory have somehow been “deported or transferred” there by the state of Israel.

Under international law, occupation occurs when a country takes over the sovereign territory of another country. But the West Bank was never part of Jordan, which seized it in 1949 and ethnically cleansed its entire Jewish population. Nor was it ever the site of an Arab Palestinian state.

Moreover, a country cannot occupy territory to which it has sovereign title, and Israel has the strongest claim to the land. International law holds that a new country inherits the borders of the prior geopolitical unit in that territory. Israel was preceded by the League of Nations Mandate for Palestine, whose borders included the West Bank. Hansell’s memo fails to discuss this principle for determining borders, which has been applied everywhere from Syria and Lebanon to post-Soviet Russia and Ukraine.

Even on its own terms, the memo’s conclusions no longer apply. Because occupation is part of the law of war, Hansell wrote, the state of occupation would end if Israel entered into a peace treaty with Jordan. In 1994 Jerusalem and Amman signed a full and unconditional peace treaty, but the State Department neglected to update the memo.

Even if there were an occupation, the notion that it creates an impermeable demographic bubble around the territory—no Jew can move in—has no basis in the history or application of the Fourth Geneva Convention. Almost every prolonged occupation since 1949—from the Allies’ 40-year administration of West Berlin to Turkey’s 2016 occupation of northern Syria—has seen population movement into the occupied territory. In none of these cases has the U.S., or the United Nations, ever claimed a violation of this Geneva Convention provision.

Mr. Pompeo’s action shows the U.S. understands that we can’t have one international law for one country and another for the rest of the world.

Mr. Kontorovich is a professor at George Mason University Scalia Law School.