Following the Vietnam-era advice attributed to Vermont Senator George Aiken that the US should declare victory and go home, Russia has decided its job in Syria is done.

Russian leader Vladimir Putin made his surprise announcement Monday that he will begin pulling most of his military out of Syria, observing on Russian state television that the move would "contribute to a peaceful resolution of the Syrian issue." It will do so if it forces Syrian tyrant Bashir al-Assad to get more serious about the peace talks with Syrian rebels. It won't, however, if it prompts ISIS and al-Qaida to move aggressively into the vacuum left by the Russian withdrawal over the next few months.

The Obama administration has regularly criticized Mr. Putin for propping up the barbaric Assad regime. This past Sunday, Secretary of State John Kerry asked the Russian leader to push his Syrian counterpart toward maintaining and securing the cease-fire agreement with rebel forces. Russia's reluctance to do so has hurt the cause of peace, but its military presence has done some damage to terrorist groups in the region.

Russia's decision gives the US and its allies an opportunity to pressure Mr. Assad to advance the peace agreement rather than sabotage it, if not to persuade him to abandon power altogether, a long-time goal of President Obama. It also means that ISIS may be able to reverse recent setbacks. Like most events in the chaotic Middle East, Mr. Putin's decision constitutes a double-edged sword.