WASHINGTON—The U.S. has quietly frozen weapon shipments to Lebanon's armed forces following the collapse of the country's pro-Western government in January, underlining growing concerns about Hezbollah's role there.

The suspension, part of a broader review of U.S. security assistance to Lebanon, offers a cautionary tale about the uncertain course of secular revolts sweeping the Middle East, and Washington's limited ability to influence them. Lebanon's Cedar Revolution of 2005 started with a wave of popular protests that stirred...