ELYAKIM, Israel — Syrian rebel groups briefly took control Thursday of the only crossing between Israel and Syria, bringing the intense violence of that nation’s civil war closer than ever to the Golan Heights, where farmers were told to stay out of their fields, tourists were turned away from cherry-picking and roads were closed.

At the same time, not 70 miles away, scores of Israeli soldiers engaged in an elaborate combat exercise preparing for what is increasingly seen here as an inevitable war with Hezbollah, the Lebanese Shiite militant group that has come to the aid of President Bashar al-Assad in Syria.

The confluence of events confronted Israel with the complex reality of a civil war just across the border in which both sides are hostile to the Jewish state. Hezbollah has vowed in recent weeks that it would facilitate attacks on Israel through the heights. And the most effective rebel force is made up of radical Sunnis aligned with Al Qaeda, while many of the other militias are led by self-identified Islamists.