Why 200 More US Troops Are Going to Syria They'll help Kurdish and Arab rebel forces retake Raqqa.

 -- Defense Secretary Ash Carter announced this weekend that 200 additional American special operations troops will be heading to Syria to help Kurdish and Arab rebel forces retake Raqqa, ISIS' de facto capital in Syria.

The deployment of the additional troops will raise the total number of American troops inside Syria to 500.

Here's what you need to know:

What Are American Troops Currently Doing in Syria?

Three hundred American special operations troops have been serving in Syria since early this year to mainly train, advise and assist the Syrian Democratic Forces (SDF), a group of 45,000 Syrian Kurdish and Arab rebel forces that have been fighting ISIS in northern and eastern Syria. A small number of these American forces have also been advising and assisting Turkish forces fighting ISIS in northwestern Syria.

Working in small teams, the special operations troops also provide explosive ordnance disposal assistance, and facilitate the communications and coordination needed for coalition airstrikes that have helped SDF fighters push ISIS out of long-held strongholds in northern and eastern Syria.

The American forces are supposed to do their work at the headquarters level, remaining behind the front lines. But given the fluid nature of the battlefield in Syria and the less structured command structure in the SDF, the American advisers can find themselves in a combat environment where they have to defend themselves if they come under attack.

The U.S. military program to assist the SDF began in early 2016 with a force of 50 special operations forces to gauge the group's fighting capabilities and reliability as a local partner in the fight against ISIS.

Based on the success of that effort, President Barack Obama ramped up the operation in April by authorizing 250 more special operations forces to go to Syria.

Why Are the Additional American Troops Heading to Syria Now?

A senior U.S. defense official said the increase will build on the momentum of the SDF and their offensive on Raqqa.

"Like our forces already in Syria, their mission will be to train, advise and assist the local forces who will take and hold Raqqa," said the official. "The increase is tied to the growing number of local forces now willing to participate in this fight, and our efforts to enable them."

Over the past year, the SDF have pushed ISIS out of the cities of Kobani, Jarabulus, Manbij and al-Shaddadi. Their success has helped the SDF grow to 45,000; and though the SDF is still mainly a Kurdish force, the number of Arab fighters affiliated with the group has grown to 13,000.

In November, the SDF began a major offensive aimed at retaking Raqqa that has brought them to within 15 miles of the city.

Today, the SDF announced that they have begun the "isolation" of Raqqa, where they will work to surround the city for an eventual push inward.

Plans call for Kurdish forces to stay out of the city while local Arab fighters will push into the city. If they are eventually successful in defeating ISIS in Raqqa, a larger number of the SDF's Arabic fighters will be required to hold the city.