Lebanese militants have been streaming into Syria in recent months to help that government in its war with the rebels—and by all accounts, they’ve been making a difference in an ongoing and critical battle in eastern Syria. Desperate to slow that tide, Syrian rebels are trying a brutal new tactic to dissuade Hezbollah militants from crossing the border: They’re decapitating captured fighters and sending the heads back home to Lebanon, according to several reports and pictures posted on various sites. One Israeli Facebook page called “News In SOS” quoted and translated Syrian rebel sources late last month saying they sent...