The Lebanese Shia Islamist militant group Hezbollah is becoming more aggressively involved in Syria’s civil war, which threatens to reopen the old wounds of Lebanon’s 1975–1990 civil war.

Hundreds of Hezbollah militants have joined the regime’s offensive to recapture the strategic Syrian town of Qusair, which sits astride an important rebel supply route from Lebanon and near the main highway connecting Damascus with Syria’s northern cities and the western heartland of the Alawite minority sect that dominates the Assad regime. Hezbollah guerillas are skilled in urban warfare and well-versed in terrorist techniques that could greatly assist Bashar al-Assad’s efforts to suppress the rebellion.

On Saturday, Hezbollah leader Hassan Nasrallah gave a defiant speech in which he finally stopped denying Hezbollah’s involvement in Syria and proclaimed, “We will be the ones to bring victory.” Nasrallah’s belligerent declaration undermined the Lebanese government’s policy of “disassociation” in the two-year-old revolt against Syria’s Assad regime.

On Sunday, Lebanon’s capital city was hit by rockets that exploded near Hezbollah’s headquarters in southern Beirut. Three Lebanese soldiers were killed Tuesday in a drive-by shooting near the border town of Aarsal, a bastion of support for Syria’s rebellion where thousands of Syrian refugees have found sanctuary.

The leader of the Free Syrian Army threatened on Tuesday to “chase Hezbollah to hell” if its forces are not withdrawn from Syria. General Salim Idriss, leader of the rebel Supreme Military Council, issued an ultimatum calling for Hezbollah to withdraw from Syria within 24 hours or face further attacks.

Hezbollah’s aggressive involvement in Syria’s civil war is ripping the scars off Lebanon’s civil war, which paralyzed the country between 1975 and 1990. Syria, which launched a military intervention in 1976 ostensibly to help stabilize Lebanon, used its military dominance to turn Lebanon into a satellite. After Syrian troops were forced to leave in 2005, Hezbollah pushed its way to power, backed by Syria and Iran. Now Hezbollah is returning the favor by supporting the beleaguered Assad regime, in close coordination with Iran.

Hezbollah’s intervention in Syria is one more reason that the European Union should impose sanctions on it and designate it as a terrorist organization. After all, Hezbollah was responsible for the deaths of hundreds of Europeans going back to the 1980s and collaborated with Iran and Iraqi Shia militias to kill U.S. and European troops in Iraq.

More importantly, Hezbollah’s escalating military support for the Assad regime should be one more factor prompting the Obama Administration to end its passive approach to the Syrian crisis. The Administration initially punted the ball to the U.N. Security Council, where Russia and China vetoed effective action, and has more recently based its policy on a regional peace conference co-sponsored by Russia that has little chance of success.

Instead of blindly trusting Moscow to help ease its only remaining Arab ally out of power, Washington should work closely with Israel, Jordan, Turkey, and other willing allies to contain the destabilizing spillover effects of the Syrian civil war, accelerate the fall of Assad, and strengthen the legitimate opposition forces inside Syria.