With tanks rolling into the border towns of Deraa governorate to quell anti-regime protests, Syria appears to have violated the 1974 Disengagement Agreement with Israel. For what it is worth, Israel is technically entitled to object to military forces and armaments deployed closer than 25 kilometres to its borders. But, in an uncharacteristic display of restraint, it has not taken any action.
It is hard to pinpoint the exact reasons for this silence, but it is a clear signal that the Israelis are cautious about upsetting the status quo as Syrian turmoil increases.
Bilateral relations are framed in part by the 1973 Arab-Israeli War. At a Geneva summit in 1974, the two countries agreed on a completely demilitarised "area of separation" along the border, which has since been monitored by the United Nations Disengagement Observer Force. For 25 kilometres in either direction, there is a further "area of limitation" that has been observed, restricting each country's military strength in the zone and generally specifying small, lightly armed forces.
On both sides there are limits on paper on the number and type of troops, tanks, anti-aircraft weapons, artillery and other weapons systems. The UN observer force inspects the area of limitation every two weeks to supervise the number and type of armaments and forces on both sides.
Recent events have clearly violated the terms of that deal. On Monday, witnesses said thousands of Syrian troops backed by tanks and a number of airplanes were deployed in various towns in Deraa. The force included the army's well-trained, notorious 4th Division. Some of the towns in Deraa - Nawa, Jasim and Enkhel - fall within the 25 kilometre-wide area of limitation.
Certainly, Israel is monitoring the situation very closely and, in turn, Syria has kept Israel in mind as it bolsters its forces near the border. By all appearances, Israel is not interested in threatening a regime that has not fired a single bullet along its border since the 1973 war. It does not take a cynic to conclude that Syria probably guessed at Israel's intentions when it gave the green light to reinforcements against protesters in Deraa.
It is very unlikely that the Syrian president Bashar al Assad sought, or would have received, explicit clearance from Israel to deploy forces to crack down on an internal uprising. The appearance of such a move would set a dangerous precedent for a Syrian regime that is already facing the biggest challenge to its existence to date. After all, Syria is the "last bastion of Arabism", at least by its own propaganda, and complicity with Israel would seriously undermine the claim.
There are at least four explanations of Israel's quiescent attitude towards its neighbour. First, Israel's increasing anxiety about an Arab street that is boiling with anger may have dissuaded it from escalating the situation. The Israeli prime minister Benjamin Netanyahu publicly expressed similar fears during the Egyptian Revolution.
A second possible reason is that Israel wants the Syrian regime to go, but knows that any intervention would be used by Damascus to justify further crackdowns and a change in attitudes among protesters. Although regime change in Syria may pose risks for Israel, a new regime might be what Israel and its allies have long wanted: a schism between Tehran and Damascus. A democratic government in the latter would probably break with Iran, especially in light of the widespread belief that Iran is assisting the Syrian regime in its crackdown against protesters. Indeed, protesters in Deraa have been shouting the slogan: "No Iran, No Hizbollah".
Another possible reason could be that Israelis are simply playing it by ear, preferring to wait until the dust settles. "Israel may calculate that the Baathist regime is on the way out regardless of what Israel does or doesn't do, and that a new regime in Damascus that is not so pro-Iranian could have potential advantages for Israel," said Dr Neil Partrick, a visiting fellow and Middle East specialist at the London School of Economics.
The fourth possible - and most probable - reason is that Israel may not be interested in the removal of a predictable partner. Working with a strongman who commands full control over the masses is, well, easier. Despite his political manoeuvring, Mr al Assad is a predictable enemy. The regime's response to every Israeli incursion has been that "Syria reserves the right to respond", or something along those lines.
Dr Mustafa Alani, a senior adviser for security and defence studies at the Gulf Research Centre, said Israel is probably aware that Syria has contravened the 1974 agreement, but is not interested in regime change. "Although it is an internal security engagement, Israel can technically interfere," said Dr Alani. "So far, I don't think they will do that. This regime gave them security for close to 40 years."
The disengagement agreement is a state-to-state agreement and only Israel or Syria - not the United Nations - can object to violations. But there is a precedent for a UN intervention that took place during the Iran-Iraq conflict. In 1988, the UN Iraq-Iran Military Observer Group identified "movement of troops and establishment of new observation posts" and called on both countries to maintain peace. That suggests that the United Nations could pressure Syria on its military actions, even though Israel is not the explicit target of troop movements.
If Mr al Assad's regime does fall, the 1974 agreement could be seen by the new leadership in Damascus as nullified, a deal between regimes that no longer existed. While Israel may be content for now to bide its time as long as this regime is in power, there are no guarantees that it will remain passive as events occur across the border.