The U.S. has around 2,200 boots on the ground in Syria, most of them allocated towards the fight against ISIS. National Security Advisor John Bolton has announced, however, that U.S. forces will remain in the country “as long as Iranian troops are outside Iranian borders, and that includes Iranian proxies and militias.” More likely to serve as a positioning or signaling message to Iran than an actual policy statement, the statement communicates America’s will to resist Iran’s attempt to fill the power and influence vacuum created by the territorial defeat of ISIS. Taking a more moderate tone, Secretary of Defense Mattis has emphasized the original intent of U.S. involvement in Syria, stating that “right now our troops inside Syria are there for one purpose, and that’s under the U.N. authorization about defeating ISIS” and underscoring the non-strategic/geopolitical elements of US involvement. Regardless of actual outcomes, U.S. policy will largely be shaped as a response to future Iranian operations and efforts in the country and broader region.
Source: Bolton: US troops staying in Syria until Iran leaves