President Trump has been chasing a diplomatic victory, and he got one this weekend when he brokered a deal between the Organization of the Petroleum Exporting Countries (OPEC) and Russia to limit their production that may also limit the bloodbath in the U.S. shale patch.

OPEC and Russia have agreed to curtail their production by 9.7 million barrels a day—about 10% of global output—in May and June amid a steep falloff in demand due to the coronavirus that is expected to exceed 20% of last year’s consumption. After Russia last...