The fundamentalist Hamas, the Palestinian group that controls the Gaza Strip, has vowed to retaliate against Israel for the killing of Mahmoud al-Mabhouh in the Gulf emirate of Dubai.
But Lebanon's Hezbollah is also bent on avenging the 2008 killing in Damascus of its leader, terrorist mastermind Imad Mughniyeh. The Mossad was blamed for that, too.
Israeli sources claim Hezbollah has made 10 attempts to attack Israeli targets inside and outside the Jewish state since Mughniyeh was blown up by a booby-trap bomb in his SUV in the Syrian capital on Feb. 12, 2008.
Israeli security authorities expect Hezbollah to try again to mark the second anniversary of the death of Mughniyeh, who until Osama bin Laden came along was the most wanted terrorist fugitive on the planet.
So the Mossad, which over the years has eliminated 20-30 of Israel's most dangerous enemies, may now find itself in a shooting war with Hamas and Hezbollah in which anything goes.
Iran has accused Israel of assassinating or kidnapping its nuclear scientists and other figures, so they're gunning for the Mossad as well.
This would seem to be pretty much business as usual in the Middle East, but it's not often that the Mossad comes under such intense antagonism from so many quarters at once.
When Mossad dispatched hit teams to kill Palestinian leaders responsible for the 1972 massacre at the Munich Olympics, it touched off a deadly war of the shadows with the Palestinians' Black September organization.
Both sides suffered losses across the Middle East and Europe.
Like many of the mysterious deaths that occur in this never-ending war, the killers of Mabhouh and Mughniyeh may never be known.
But perception is often more important than substance in this high-tension environment and the Mossad, which cloaks its clandestine operations in deliberate ambiguity, counts on its fearsome reputation for ruthlessness in hunting down Israel's enemies to intimidate them.
The details of the Mabhouh slaying in his room at the luxury Al Bustan Hotel in Dubai remain murky. His body was found by hotel staff after lunch on Jan. 20 and it was thought he'd died of a heart attack.
But nine days later, after medical analysis showed traces of poison in his blood, Hamas announced that he had been tortured with electric shocks in his hotel room and then injected with a drug that induced a heart attack.
Hamas officials accused Israel of killing Mabhouh, a founder of the movement's military wing, the Izzedine al-Qassam Brigades.
The Israelis have killed dozens of top Hamas men over the last decade, but all were slain in the Gaza Strip, usually in airstrikes.
As far as is known, no Hamas operative has been targeted outside Gaza, although militants from other groups have.
So if the Mossad was behind Mabhouh's death, it marks a new departure and possibly a new clandestine offensive against the militants' leadership.
Mossad has used similar tactics before. Mossad agents jumped Khaled Meshaal, Hamas' politburo chief, outside his office in Amman, Jordan, in September 1997 and sprayed a chemical in his ear.
He fell into a coma and would have died but for King Hussein of Jordan.
The monarch demanded that Mossad provide the antidote and threatened if it did not he would tear up his 1994 peace treaty with Israel.
That was one of the very few occasions where the Mossad was forced to go public over an assassination mission.
Mabhouh's family said he had survived an assassination bid in Gaza in 1989, and another in Beirut last summer when he was poisoned and spent 36 hours comatose.
Israeli officials say that Mabhouh, a veteran Hamas operative well versed in operating clandestinely, was in charge of procuring weapons and explosives for Hamas, which receives arms and funding from Iran.
Officials in Dubai said Mabhouh arrived there on Emirates flight EK912 from Damascus on Jan. 19.
They say he was tailed by at least two men carrying European passports who were members of a "professional criminal gang" who quit Dubai right after the killing -- all hallmarks of a Mossad operation.