BREMMER: Putin is serious about entering Syria - and not to go after ISIS

The regime of Syrian President Bashar al-Assad is weakening, and one of his primary backers - Russian President Vladimir Putin - is upping the ante.

"Assad has lost significant territory over the past months; Putin is not about to tolerate his ouster," geopolitical expert Ian Bremmer, the president of Eurasia Group, told Business Insider in an email.

The regime has recently lost significant territory to al-Qaeda-led rebels in the north, ISIS in the country's center, and nationalist rebels in the south.

The West, meanwhile, is ramping up air operations against ISIS as throngs of Syrian refugees flee Assad's barrel bombs and attempt a daunting trip to Europe.

Western actions that bolster rebel forces in the north - where they are fighting the regime in Syria's largest city of Aleppo and ISIS in the countryside - and weaken Assad further could contribute to a settlement to end the war. This is where Putin becomes worried.

"If the West succeeds in turning the tide of the war while Assad is vulnerable, the political outcomes in Syria are more likely to be dictated by the US," Bremmer said. "Which means Putin needs to bolster Assad now."

And Russia seems to be doing just that: Russian military experts in Syria are inspecting and enlarging airbases. Others are setting up housing units for up to 1,000 personnel. Advisers are meeting with Iranian and Syrian counterparts in the capital. Russian drones and fighter planes are surveilling non-ISIS rebels in the country's north. And Russian armored personnel carriers with Russian-speaking troops are involved in fighting.

Furthermore, given his priorities, Putin isn't too worried about ISIS.

Russia's surge in support is "less likely to mean helping the Assad regime combat ISIS directly - that's expensive and a job that the Russians would rather see the West take on (and suffer the consequences of)," Bremmer said. "But rather to best position Assad for the eventual terms of a weakened or post-ISIS Syria."

Emile Hokayam, a Middle East analyst at International Institute for Strategic Studies, noted on Twitter that Russian and Iran know that without Assad, "there is no regime in Syria to secure their interests."

Consequently, he added, it doesn't matter if they like him - because he's the "only game in town."

And Russia is not worried about the West, given that the US has largely avoided Syria and Europe is already looking for a way out of sanctions against Russia over Ukraine.

"Clearly Putin's not particularly bothered by continuing to frustrate the United States," Bremmer said. "And the Europeans aren't going to punish him for military engagement in Syria - they're more interested in coming to terms with Assad just as they're more prepared to see a frozen conflict in Ukraine (see Hollande's comments on his hopes to end sanctions)."

The fallout of all of this, according to Bremmer, will be more chaos - and more refugees headed to Europe.

"As the West presses ISIS while Russia provides direct support for Assad, the Syrians are caught in the middle.," Bremmer said. "Anyone that can find a way out will. And the Syrian refugee crisis in Europe, four years in the making, is guaranteed to expand for a fifth."