For the special relationship, a Corbyn government would do the most damage in the arena of national security. Consider the possibility that Corbyn could win a national election in 2019. In such a situation, the U.S. government would probably scale back its national-security cooperation with Britain in a wide range of areas. From the NSA’s vast signal-intercept cooperation with its British equivalent (Government Communication Headquarters), to the CIA and FBI sharing of reports by agent (covert sources inside terror groups and hostile governments) with MI6 and MI5, the special relationship would take a big hit. Foreign intelligence partnerships require trust. But a Corbyn government would fundamentally undermine trust: Because of Corbyn’s evident sympathy for Iranian and Russian foreign policy, the U.S, could not trust him with U.S. intelligence sources that he then might leak to Putin. And if it lacked access to U.S. intelligence, Britain would be less able to defend itself against enemies at home and abroad. Even limited restrictions on U.S. counterterrorism intelligence-sharing would pose a serious challenge in Britain’s current threat environment.

But it’s not only in the matter of intelligence that a Corbyn government would wreak havoc for the special relationship. Traditionally, Britain has been America’s geo-strategic partner. This partnership affords both nations great influence in the EU. Knowing that the American president has the ear of the British prime minister — and vice versa — German and French leaders understand they must often placate the British government and listen to British representations of American concerns. But think for a short second: Would any American president ever trust Jeremy Corbyn?