BEIRUT — Retaliatory Turkish artillery strikes deep into Syria have showed the speed with which the bloody civil war can entangle its neighbors and destabilize an already volatile region.
Beyond the cross-border flare-up, the 18-month battle to oust Syrian President Bashar Assad has already deepened sectarian rifts in Lebanon and Iraq, raised tensions along the long quiet frontier with Israel and emboldened Kurdish separatists in Turkey.
"There is not a single country bordering Syria that we can honestly say they are not facing a realistic threat to internal stability and national security," said Aram Nerguizian of the Center for Strategic and International Studies in Washington.
From the start, Syria's conflict burst over its borders. Hundreds of thousands of Syrians sought refuge across the country's borders with Jordan, Turkey, Lebanon and Iraq. Stray bullets and mortar rounds, sometimes with deadly result, have struck Turkey, Jordan, Lebanon and the Israeli-annexed Golan Heights.
But in a dramatic escalation on Thursday, Turkey fired back for the first time after an errant Syrian mortar shell killed five people in a Turkish border town Wednesday. Turkey shelled Syrian military targets, and Turkey's parliament approved future retaliation under such circumstances.
Turkey said it did not amount to a declaration of war, and Syria offered a rare apology — a sign that both want to defuse tensions.
Assad's foes, including Turkey, have been unwilling to intervene directly in Syria, and the Damascus regime has tried to make sure it stays that way, avoiding major provocations that could inadvertently trigger foreign intervention.
With Thursday's parliament decision, Turkish leaders expanded their options for dealing with Syria but avoided a full-scale military confrontation, said Ayham Kamel, a Middle East analyst at the Eurasia Group in London.
Some of Syria's other neighbors, particularly Lebanon, have also shown restraint, in part to try to avoid inflaming sectarian divisions within their own countries that mirror the divides in the Syrian civil war.
Many of those rising up against Assad are Sunni Muslims, while Syria's ruling elite is dominated by Alawites, an offshoot of Shiite Islam. Syria's Christian and Kurdish minorities have largely been trying to stay out of the line of fire.
Both Iraq and Lebanon have an ethnic and religious mix similar to Syria's, and in Lebanon, sectarian tensions have been rising. Since May, repeated street clashes between pro- and anti-Assad groups in Lebanon's majority Sunni port city of Tripoli have killed more than two dozen people.
In a further complication, the pro-Assad Hezbollah militia, which is believed to be sending fighters to help the embattled Syrian regime, is a major political and military force in Lebanon.
However, Lebanon's major players have largely resisted the temptation to exploit the Syrian conflict for political gains at home, several analysts said.
Lebanon's 15-year civil war, which ended in 1990, is still deeply etched in the collective memory, and there seems to be little appetite for another round.
"I don't think anyone in Lebanon, among the major political leaders, the major political factions, wants to support a sectarian war (at home)," said Michael Young, the opinion editor for Lebanon's Daily Star newspaper.
But Lebanon's fragile coalition government may not be able to withstand a protracted conflict next door, and any miscalculation could ignite violence.
"What the crisis is doing is eroding the resilience of the state," said Emile Hokayam of the International Institute for Strategic Studies, a London-based think tank. "Small security incidents may escalate just because of bad management."
For Iraq's Shiite-led government, the Syrian civil war has made the job of balancing the demands of Baghdad's main patrons, the U.S. and Iran, even more difficult.
Last month, Iraqi Prime Minister Nouri al-Maliki came under pressure from Washington to ban Iranian planes suspected of carrying weapons to Syria from using Iraqi airspace. Sticking to official neutrality on Syria, al-Maliki said he'd try, but that Iraq could at best perform spot checks.
The fighting spirit of Syria's Sunnis, meanwhile, has helped embolden a Sunni insurgency in Iraq that had been withering for years. If Assad is defeated and Syria joins a Sunni coalition in the region, Iraq might find itself seeking even closer ties with Iran.
Turkey's outspoken support for the Syrian rebels — a policy it adopted in August 2011, after trying to reason with Assad first — has coincided with a sharp rise in increasingly brazen attacks by the separatist Kurdistan Worker's Party, or PKK, in southeastern Turkey.
In August, Turkey's deputy prime minister, Bulent Arinc, said a number of those attacks were apparently carried out by Kurdish gunmen who infiltrated from Iran, a staunch Assad ally. A Turkish pro-government newspaper, Sabah, has claimed that Iranian intelligence has stopped providing Turkey with information about Kurdish rebel infiltrations.
Assad has also used Syria's Kurds as pawns against Turkey.
After Turkey repeatedly raised the idea of internationally enforced safe zones in Syria in the spring, Syrian regime forces withdrew from several Kurdish towns on the border with Turkey over the summer. That granted them unprecedented autonomy, but also set them up as a buffer zone.
Assad was telling Turkey: "'You can intervene (in Syria), but you're going to have to fight the PKK on the way to get there'," said Soner Cagaptay, a Turkey expert at the Washington Institute for Near East Studies.
Picking a side in Syria also seems to have ended a decade-long effort by majority Sunni Turkey to increase its regional influence by getting along with all players, including Iran.
In Jordan, the biggest problem for now seems to be the strain put on the country's meager resources by some 200,000 Syrian refugees who have flooded across the border.
Israel has tried to stay on the sidelines, but last month sent a clear message that it's prepared for all scenarios. In a snap military drill, it airlifted thousands of soldiers to the Golan Heights, a strategic plateau Israel captured from Syria in 1967, and carried out live fire drills there.
Israel could face a serious threat to its security if Syrian territory near the Golan becomes a chaotic no-man's land, said Giora Eiland, a former national security adviser in Israel. Last week, Syrian mortar shells fell near a Golan apple orchard, but Israel said it did not believe it was an intentional hit.
However, Iranian advisers are helping direct the Syrian regime's battles, the defected Syrian prime minister said last month. This would put them just a frontier away from Israel at a time of increasingly ominous threats between Israel and Iran over Tehran's suspected nuclear weapons program.
Nerguizian, the Washington analyst, said Syrian conflict has set in motion sweeping changes that will transform the region.
"At this rate, it is not going to look anything like the pattern we've seen in the past few decades, since the end of World War II," he said.
Associated Press writers Suzan Fraser in Ankara, Turkey, Barbara Surk in Beirut and Lauren E. Bohn in Jerusalem contributed reporting.