In December 2015, as David Cameron's government prepared to strike Isis in Syria, Labour was dramatically split. Jeremy Corbyn opposed the intervention but his shadow foreign secretary Hilary Benn and 10 other shadow cabinet ministers supported it.

Donald Trump's decision to launch airstrikes against Bashar-al Assad has left Labour similarly riven. As the Conservatives and the Liberal Democrats backed the action, the media waited for an opposition response. But the first shadow cabinet member to react was not Corbyn or his shadow foreign secretary Emily Thornberry but Tom Watson. After being called by the Birmingham Post and Mail's political edtior Jonathan Walker, Labour's deputy leader backed the strikes, calling them "a direct and proportionate response to a clear violation of international law by the Syrian regime".

Watson, who personally assured Cameron in 2015 that he would support intervention, added: "It's clear from the nerve gas attack in Khan Sheikhoun earlier this week that President Assad had retained a chemical weapons capability, contrary to what was agreed in 2013.

"Indiscriminate chemical weapons attacks on civilians can never be tolerated and must have consequences. It's vital that the United States is now clear about its intentions and that the whole international community works towards a political settlement in Syria."

As journalists continued to wait for Corbyn's response, other Labour voices backed the air strikes. The party's backbench defence committe said: "The US action overnight was proportionate and should have Labour's full support." Hilary Benn tweeted: "Let's hope Syria will now think twice before deciding to gas its own people again. Priority must be humanitarian assistance for civilians."

At 11:19am, more than three hours after the Conservatives and Lib Dems had responded, Corbyn issued a statement (on which Watson was not consulted). "Stop criticising Corbyn's slow response," tweeted Labour MP Michael Dugher, who was sacked from the shadow cabinet last year. "It takes time for Seamas to run the draft statement by the Kremlin, Stop the War + the Morning Star".

As expected, Corbyn, who has opposed every western military intervention in recent history, did not support the strikes. He warned that "unilateral military action without legal authorisation or independent verification risks intensifying a multi-sided conflict that has already killed hundreds of thousands of people". Though his condemnation of the US was more restrained than in the past, his position was unambiguous.

Corbyn's stance has left the shadow cabinet once more hopelessly divided. Nia Griffith, the shadow defence secretary, was consulted and expressed her support for military action before her leader opposed them. Labour sources estimate that most MPs and at least half of the shadow cabinet back the strikes. Corbyn has been left appearing isolated as no other senior figure has publicly echoed his stance.

It is on foreign policy that the Labour leader's views are more determined than any other subject. Corbyn, who has long deferred to shadow chancellor John McDonnell on economics, has spent much of his campaigning life denouncing western military action. Until 2015, he was chair of the Stop the War Coalition, a group reviled by most Labour MPs. As long as Cobyn remains leader, his party and shadow cabinet will be fatally split on foreign policy.