A look at the reasons for and possible implications of the escalation of Israel's involvement in Syria's civil war.
Israel has said repeatedly it does not want to get dragged into Syria's civil war but has also warned that it will not allow so-called "game-changing" sophisticated weapons to flow across the border to Lebanon's Hezbollah, an Islamic militant group allied with the Syrian regime.
Israeli defense officials believe Iran has stepped up shipments of weapons to Hezbollah through Syria, including accurate longer-range Iranian missiles, as President Bashar Assad's position weakens. This could help explain the back-to-back Israeli strikes on Friday and Sunday on alleged Hezbollah-bound weapons in Syria. Before this week, Israel aircraft had struck Syria only once, in January.
Analyst Paul Salem of the Carnegie Middle East Center in Beirut says Israel may simply be sending a stern warning to deter such weapons smuggling. Salem says Israel also appears to be increasingly concerned about Iranian and Hezbollah forces fighting alongside Assad's troops, close to Israel's borders.
White House spokesman Josh Earnest said Sunday that "Israelis are justifiably concerned about the threat posed by Hezbollah obtaining advanced weapons systems, including some long-range missiles."
President Barack Obama said the U.S. coordinates closely with Israel, implying that Washington was not taken by surprise by the Israeli strikes.
The U.S. has long resisted getting involved in the conflict amid concerns that foreign weapons could end up in the hands of al-Qaida-linked groups or other extremists fighting with the rebels. However, Defense Secretary Chuck Hagel said recently that Washington is reviewing its opposition to arming the opposition.
The Israeli strikes illustrate that Obama and Israeli Prime Minister Benjamin Netanyahu have drawn different red lines in Syria's conflict.
Israel's main concern is that Hezbollah could obtain advanced weapons.
Obama has warned that the use of chemical weapons by the regime could have "enormous consequences." There have been some indications of chemical weapons use, but Obama has said he needs more definitive proof before making a decision about how to respond.
Israel, which commands the region's most powerful military, appears to be taking a calculated risk that Syria, Hezbollah or Iran won't retaliate for its air raids.
If the trio were to do so, it would mean opening up a new front at a time when it is fighting for the Assad regime's survival. Hezbollah also risks losing its position as the dominant military and political power in Lebanon, something it painstakingly rebuilt after the 2006 war, if Israel were dragged in to the conflict.
The initial Syrian response to Israel's airstrike early Sunday appeared relatively muted. Syria's government called the attacks a "flagrant violation of international law" and warned it has the right "to defend its people by all available means."
Still, Israeli officials have signaled Israel will not stop blocking weapons shipments to Hezbollah, raising the possibility of more Israeli airstrikes and a further escalation. Salman Shaikh of the Brookings Doha Center in Qatar says the Assad regime, Hezbollah and Iran increasingly view the Syria conflict as a zero-sum game.
Iran is the senior partner in the axis since it supports the Assad regime and Hezbollah with weapons, though it's not clear how much tactical sway is being exerted by Tehran.
Advisers from Iran's Revolutionary Guard are believed to have longtime roles in Hezbollah's militia forces and Assad's army — serving as both point men for Tehran's aid and liaisons with the ruling clerics in Tehran. Yet Iran also keeps a distance from the actual battlefield.
Gen. Masoud Jazayeri, assistant to the Iranian chief-of-staff, told Iran's state-run Arabic-language Al-Alam TV on Sunday that Tehran "will not allow the enemy (Israel) to harm the security of the region" and that "the resistance will retaliate against the Israeli aggression against Syria."
Iran would have a major say in any decision to retaliate for the airstrikes but is not believed to have an appetite for a confrontation with Israel. While Iran is fighting for regional influence and has often used its anti-Israel stance to do so, it has never attacked the Jewish state.
The uprising against Assad erupted in March 2011 and quickly evolved into a civil war, leaving tens of thousands dead and millions displaced. Assad and those trying to topple him remain locked in a battlefield stalemate, with neither side able to deliver a decisive blow.
The rebels are dominated by Syria's Sunni Muslim majority, while Assad is a member of the Alawite sect, an offshoot of Shiite Islam. He as rallied hardcore supporters around him, including members of Syria's ethnic and religious minorities who prefer the current regime to Sunni majority rule.
During four decades of rule, Assad and his father and predecessor, Hafez, used Syria's staunchly anti-Israel positions as a source of legitimacy even though both men kept Syria's frontier with the Jewish state quiet.
Syria's civil war has increasingly eroded Assad's anti-Israel "credentials," after the regime attacked Palestinian refugee camps in Syria and the Palestinian militant group Hamas, made up of Sunnis, broke with the regime because of its crackdown on the rebels.
If Assad does not retaliate for the latest Israeli strikes, his claims to anti-Israel militancy would become even more tenuous. In an attempt to deflect attention, Syrian government officials on Sunday tried to portray the Syrian opposition as engaged in a common cause with Israel.
Future Israeli air attacks could also wipe out key Syrian military installations. Rebel forces have managed to seize a number of Syrian military bases, seizing heavier weapons but have advanced only slowly because of the regime's air superiority.