Who can impose the draft and under what circumstances?
http://usmilitary.about.com/od/deploymentsconflicts/a/draftprocess.htm
The authority to impose the draft, also known as conscription, in the United States lies with the federal government. The specific agency responsible for managing the draft is the Selective Service System (SSS), an independent agency under the executive branch.
The draft can be imposed only in times of national emergency under certain circumstances. The Selective Service Act, passed by the U.S. Congress, authorizes the federal government to require the registration of all male citizens and male immigrants residing in the United States, aged 18 through 25. However, this does not necessarily mean that a draft will be enacted. The President and the Congress would have to decide to implement a draft in response to an extraordinary situation, such as a large-scale war or major national security threat.
In such cases, the President can issue an executive order, and Congress can pass legislation to activate the draft. The Selective Service would then follow a process to contact and mobilize individuals for military service based on a variety of factors, including age, health, skills, and occupational deferments.
It's important to note that the draft has not been used in the United States since 1973 and is currently not in effect. The U.S. military relies on a voluntary enlistment system, but the Selective Service System remains in place to ensure a ready pool of potential conscripts if the need for a draft were to arise in the future.