WASHINGTON — Purse snatching and pickpocketing can amount to violent felonies for purposes of a federal law, the Supreme Court ruled on Tuesday in a 5-to-4 decision featuring unusual alliances.

The case concerned the Armed Career Criminal Act, a federal law that is a kind of three-strikes statute. It requires mandatory 15-year sentences for people convicted of possessing firearms if they have earlier been found guilty of three violent felonies or serious drug charges.

Figuring out what qualifies as one of those earlier offenses is not always easy. Tuesday’s decision considered a part of the law that defined violent felonies to include offenses involving the use or threat of physical force. The question in the case was whether minimal force, as in a purse snatching, is enough.

In analyzing whether given crimes qualify as violent felonies under the federal law, the Supreme Court does not look to what the defendant actually did. Rather, it considers whether the crime — in this case, robbery under Florida law — covers conduct that does not qualify as a violent felony.