Sharif Nashashibi Al Arabiya It was always inevitable that Syria's revolution would have repercussions in neighboring Lebanon, but since the start of the uprising two years ago, never have we come closer to all-out, direct conflict between Syrian and Lebanese forces. The rebel Free Syrian Army has issued an ultimatum to Hezbollah, giving it 48 hours to cease its operations in Syria, or face attacks in its stronghold of southern Lebanon. In response, Hezbollah - a staunch ally of the Syrian regime - has reportedly begun sending reinforcements to the border. This alarming escalation comes just days after clashes resulted in the killing of three Hezbollah fighters and the wounding of 14 others, as well as five FSA fatalities. The fighting reportedly broke out after Hezbollah forces, who have been in control of eight Syrian border villages since last year, tried to move into three adjacent villages that are in FSA hands. This is a “serious threat to Syrian-Lebanese relations and regional peace and security,” said the Syrian National Coalition, adding that it holds the Lebanese government responsible for ending the “aggression.” The opposition umbrella group is perfectly justified in its condemnation of Hezbollah's inexcusable and inflammatory presence in Syria. The Lebanese movement's leader Hassan Nasrallah admitted in October 2012 that party members had fought Syrian rebels, but his claim that they were acting individually, not under his direction, is nonsense. There is no way that Hezbollah fighters would be directly involved in the Syrian conflict against the wishes of such a well-organized, tightly knit party. Nonetheless, the FSA's ultimatum is a major miscalculation. It is a threat it cannot successfully carry out, and one it can ill afford to attempt. It is unlikely that Hezbollah will acquiesce; sending reinforcements to the border indicates the contrary. Pulling out of Syria in the face of such an ultimatum would be an embarrassing climb-down for a movement that prides itself on strength and defiance. As such, if Hezbollah ignores the FSA's demand, the Syrian rebels will themselves look weak if they do not make good on their threat. However, entering Lebanon to fight Hezbollah would be disastrous. The FSA is no match for its proven military prowess, particularly on its home turf. If Israel, whose army is the strongest in the region, has been incapable of defeating it, what hope for Syrian rebels? Hezbollah has been shaken by the revolution against its ally in Damascus, and its traditional regional popularity has been battered because of its hypocrisy in backing other Arab revolutions but not Syria's. However, this is unlikely to have affected its military power significantly. The FSA has been unable to oust Bashar Assad despite two years of fighting. Opening up a second front against another strong foe would make this goal exponentially harder, if not impossible. Furthermore, there is no indication yet that other rebel forces would join such a mission, and if foreign material backing against the Damascus regime has been lacking, it would be woefully naive to expect such support in this case. It might also give Hezbollah, and perhaps Iran, an excuse to increase direct involvement inside Syria in aid of Assad. Extending the conflict to Lebanon would be considered foolhardy by Syrians and others who support the revolution, thereby affecting the FSA's vital domestic support and foreign backing. It would also be deeply unpopular among all Lebanese, even those who support Syria's uprising. After all, Lebanon's March 14 alliance, which backs the Syrian opposition, was behind the “Cedar Revolution” that resulted in the withdrawal of Syrian troops in 1995, after a 30-year presence and domination of the country. Although there is no love lost between the alliance and Hezbollah, the former would not, and could not, support the idea of Syrian fighters in Lebanon, for whatever reason. For the sake of the revolution, those within the Syrian opposition, as well as their foreign backers, must urge the FSA not to enter Lebanon. Likewise, if Assad's allies have any sense, they would urge Hezbollah to pull out of Syria. The rebels are well within their rights to repel Hezbollah, but only on their own soil. The next 48 hours may be another dark turning point in the escalation, and possible extension, of the Syrian conflict. — Sharif Nashashibi, a regular contributor to Al Arabiya English, The Middle East magazine and the Guardian, is an award-winning journalist and frequent interviewee on Arab affairs. He can be found on Twitter: @sharifnash