Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is completing a book about President Trump's national security team and policies. The opinions expressed in this commentary are his own. View more opinion articles at CNN.

Not much will bring senior Republicans to push back against Trump, but his decision to pull American forces out of Syria has drawn condemnation from three grandees of the GOP: Sen. Lindsey Graham , Senate Majority Leader Mitch McConnell, and former UN Ambassador Nikki Haley.

Kurds say US has betrayed them, says former US diplomat

Kurds say US has betrayed them, says former US diplomat 01:26

Kurds say US has betrayed them, says former US diplomat

And for good reason: Pulling the 1,000 or so American forces out of Syria makes no sense.

Those troops are not there on a combat mission but only in an advisory role to prevent the return of ISIS and also to provide some US leverage over events in Syria, whose regime is supported by Russia and Iran.

An American pull-out also allows the Turks free rein to carry out military operations against America's allies in the largely Kurdish Syrian Democratic Forces (SDF), which were the ground troops that retook Syria from ISIS. The SDF is regarded as a terrorist group by Turkey, which has a restive Kurdish minority.

But the SDF have been the best kind of allies to the United States, losing, according to a statement from the SDF, some 11,000 soldiers fighting ISIS. This, while the US has had 17 soldiers killed in action in both Syria and Iraq during the past five years.

What is particularly odd is that we have already seen this movie before from Trump. Last December, President Trump ordered the withdrawal of the 2,000 US ground troops then in Syria.

This precipitated the resignation of the then-Secretary of Defense, James Mattis, who saw the move as an abandonment of American allies on the battlefield. Eventually, Trump was talked out of a total US withdrawal from Syria because of the possibility that ISIS might return, and because it would give Iran greater sway in the country.

If there is one key lesson learned from fighting jihadist terrorist groups in the 18 years since 9/11, it is that they thrive in weak or failing Muslim states such as Syria, and other countries where the United States has little to no presence.

The US has run a version of this play before. After the United States pulled out of Iraq, the group that later became ISIS first organized in Syria in 2011, before it invaded Iraq three years later and took over much of the country.

What is particularly odd about Trump's policy shift on Syria is that it does exactly what Trump repeatedly warned against during his presidential campaign: it gives America's enemies an early heads-up about US military plans.

During his campaign, Trump also had correctly critiqued the total American troop withdrawal from Iraq under Barack Obama in 2011 as helping pave the way for the rise of ISIS.

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What Trump's announcement on Syria also underlines is the collapse of any kind of deliberative national security process. Trump appears to make unilateral decisions surrounded by a cabinet of yes-men and acolytes because he has chased out advisers like Mattis, former National Security Adviser H.R. McMaster, former Secretary of State Rex Tillerson, and former Chief of Staff John Kelly, all of whom tended to push back against Trump's more ill-advised decisions.

Such as a precipitous withdrawal from Syria.