Investigators announced this week that they'd concluded that Hezbollah, a Lebanon-based anti-Israeli militant group, was behind a 2012 bombing that killed five Israeli tourists in the resort town of Burgas. The United States, which considers Hezbollah a terrorist group, responded by pressing the European Union to designate it as such.

Thanassis Cambanis, a journalist who has covered Hezbollah from within Lebanon and wrote an excellent book on the group, argues at that this could represent the beginning of a larger shift for them. "The Party of God, once eager to forswear tactics considered terrorist, appears to be tilting back into their embrace."

The deliberate investigation of the Bulgaria bombing could heighten alarm about Hezbollah. Last year Hezbollah was accused in plots against Israeli targets around the globe, some foiled ahead of time (Bangkok, Baku), some botched in execution (India, Georgia), and some lethal, like the one in Bulgaria. Perhaps Hezbollah is frustrated by its own weakness; since 2008, it has sworn to retaliate against Israel for the assassination of Hezbollah's military mastermind, Imad Mughniyeh. Yet more than four years have passed, and Hezbollah has failed in any plots against high-profile Israeli targets. Hence, perhaps, the targeting of Israeli holidaymakers instead.

Thanassis says that Hezbollah "faces tremendous pressure as its patrons in Syria are on the verge of a state collapse," which might help explain why it would return to the terrorist-style tactics that it had previously tried to avoid. He writes, "the more threatened Hezbollah and the [Sryrian President Bashar] Assad regime feel, the more likely they are to make wild moves." That doesn't mean that they necessarily will, but it's a potentially worrying change in the group's calculus.