PARIS—The head of the U.K.’s Secret Intelligence Service urged the U.S. not to allow Russia to succeed in Ukraine, warning that it would embolden other authoritarian states across the globe and jeopardize trans-Atlantic security.

Richard Moore emphasized the risks of not aiding Ukraine, warning that its defeat could embolden threats from Russia, China, and North Korea.

The US and Europe face a breakdown in transatlantic security if they stop supporting Ukraine’s war against Russia, the head of Britain’s foreign intelligence agency warned in his first public remarks since Donald Trump’s election win.

Richard Moore, the chief of the British Secret Intelligence Service, known as MI6, believes that if Russian President Vladimir Putin manages to achieve victory and turn Ukraine into a vassal state, he will not stop there,