Not just the withdrawal of troops, but the betrayal of the Kurdish militias who've largely served as the foot soldiers in the war against ISIS, supported by US air and artillery support and intelligence - which will all be gone in 30 days.
And Trump's continued ignoring of his intelligence and military advice (including Mattis' on repeated occasions). Trump wants a great big shiny military for parade purposes only.
The foreign-policy-by-impulsive-tweet, with zero consultation with US allies (the British, French, Germans, Australians, Canadians, all the rest of NATO, US Congress, US DOD, VP Michael Pence, Don Jr., Ivanka, etc. - hell everyone on earth apparently) all found about the withdrawal order via Trump's Twitter account. 48 hours later, there still has been no official informing of US allies. Good luck on any future alliances with the Trump administration. The issue of trust has been abandoned. Even pro-Trump Israel is shaken by this action.
ISIS can now be expected to rebuild and try to retake territory they were kicked out of over the last 4 years. They'll find no US drones in the sky, no US artillery strikes, no US Special Forces opposition, etc. Many of the same Americans who were extreme in their denunciation of Obama for insufficient use of the military to crush ISIS will now look at the ground, shuffle their feet and mumble something about low unemployment rates and tax cuts.
I'm hearing a very small minority of elected GOP leaders offering support of the Syria withdrawal, and a whole lot of them lamenting (but understanding) the resignation of General Mattis.
The Iranians are thrilled. So is Hezbollah. So is the Syrian government. So is, especially, Russia who is filling the vacuum left by the US withdrawal.
Without question, the next Secretary of Defense will be a MAGA-ite figurehead whom will never, ever speak a syllable of dissent to the current Oval Office occupant and lack Mattis' full throated opposition to Russia.