After nearly a year in which Russia seemed content to be a bystander in the Middle East as a US-led coalition did battle with the militant group Islamic State in Iraq and Syria (ISIS), Moscow has suddenly and swiftly ramped up its involvement.

This week, it apparently caught United States officials by surprise when it commenced air strikes in Syria, reportedly giving American forces only one hour's notice to stay out of its way.

It is thought that Russia grew concerned that Syrian President Bashar al-Assad's government would collapse and either wanted to shore up its ally or make sure it had a hand in picking a successor. But the situation has forced the United States to reassess its approach to the fight against ISIS.

Russia has changed the equation in two important ways for the US. The more immediate problem is the risk of an accidental military clash with Russia. Both US and Russian planes are now launching daily air strikes in the crowded Syrian airspace. Worse, the two sides do not agree on which areas to target. The US has accused Russia of hurting civilians and bombing anti-Assad rebels not linked to ISIS.

Talks aimed at "deconflicting" - or efforts to ensure forces from both sides don't shoot at each other - also appear to be going nowhere.

The second is that Russia's deployment seriously undermines what is left of the US strategy in the region. The US has insisted that Mr Assad's removal is a necessary step in defeating ISIS while Russia takes the opposite stand. The events of this week have made it harder to remove the Syrian leader.

Observers say the US may be willing to accept a deal with Russia where Mr Assad stays in power during a transition. But it is unclear whether the Americans will be able to sell such an outcome to its coalition partners, especially regional players like Saudi Arabia that insist the Syrian leader must leave.

The Russians are proving to be a game changer in the region.