Russia has used a military-grade terror weapon in an assassination on the soil of America’s closest ally. Washington must stand firmly with London in retaliating against the Putin regime.

The nerve agent that brought former Russian spy and British double agent Sergei Skripal and his daughter, Yulia, to death’s door is one of the Novichok family of toxins developed by the Soviet Union in its last days.

Unless the Russians can show they somehow lost control of highly secret weapons, British Prime Minister Theresa May told the House of Commons, this was “a direct act of the Russian state against our country.”

Neither May nor President Trump can hide behind courtroom standards of evidence here: Someone Putin wanted dead was attacked with a weapon only he has — an assault that also exposed thousands of Britons to the poison and forced dozens to get medical treatment.

This sort of “brazen act to murder innocent civilians on our soil,” will not stand, May said.

If Moscow doesn’t provide an explanation, she will “conclude that this action amounts to an unlawful use of force by the Russian State against the United Kingdom,” and respond accordingly.

Team Trump needs to back her play — indeed, to make sure she doesn’t go wobbly. For starters, both nations can target the billions that top Putinites now much-too-easily park in the West.

More active responses must follow. Putin has crossed a line that the Soviets honored even in the depths of the Cold War. If he gets away with it, there’s far worse to come.