The situation in Syria just took a huge turn. Anti-regime rebels have launched a fast and unexpected push south, shaking the entire region. This sudden move has placed Syrian President Bashar al-Assad’s grip on power in serious jeopardy.

The rebels pushing to overthrow Assad still have a long way to go, but they’ve already made significant progress, especially in Syria’s northwestern regions. This is a huge problem for one of Assad’s most important allies—Russia. Even though Assad’s rule...