The key players in the deployment talks

Theresa May, then UK prime minister

Mrs May was acutely aware of the dangers posed by Isil fighters if they were left free to roam in Syria, given her previous role of home secretary. She conferred with trusted advisers but eventually ruled out a major surge in British troops on the ground.

Gavin Williamson, then UK defence secretary

pictured, had a strong relationship with his US counterpart and is understood to have been provisionally supportive of increasing troop numbers. He was in Ukraine after Mr Trump’s announcement but stayed in touch with UK and US figures.

Donald Trump, US president

Mr Trump announced the withdrawal of 2,000 US troops a week before Christmas 2018, tweeting it was

“time to bring our great young people home”. He saw it as delivering on his promise to terminate “endless wars”, but it prompted a backlash from US military chiefs and senior Republican Party figures.

Jim Mattis, then US defence secretary

Mr Mattis was incensed by Mr

Trump’s move, seeing it as disastrous for US interests in the region and undercutting the country’s allies, including Britain.

He went to the White House with a resignation letter and would be out of the job days later. But Mr Trump eventually did give in to the pressure at home and left 1,000 US troops in Syria.

Emmanuel Macron, French president

Mr Macron led the way in convincing Mr Trump to join Britain and France in bombing Syrian government targets after suspected chemical weapons use. Under the UK plan, France would have split the burden of replacing US troops on the ground. It is not known if Mr Macron was consulted before the proposal was dropped.