U.S. officials believe members of Hezbollah, the militant group backed by Iran, are smuggling advanced, guided-missile systems into Lebanon from Syria piece by piece to evade a secretive Israeli air campaign designed to stop them.

The moves illustrate how both Hezbollah and Israel are using Syria’s civil war as cover for what increasingly is seen as a complex and high-stakes race to prepare for another potential conflict—their own—in ways that could alter the region’s military balance.

Some components of a powerful antiship missile system have already been moved to Lebanon, according to previously undisclosed intelligence, while other systems that could target Israeli aircraft, ships and bases are being stored in expanded weapons depots under Hezbollah control in Syria, say current and former U.S. officials. […]