Israel and Syria signed a disengagement agreement in 1974 to end the Yom Kippur War. It has held for a remarkable 50 years.
Now, it appears that Israel is violating that agreement by entering areas that it was not allowed to enter under the agreement.
Is this legal?
First of all, the question is whether the Syria that signed the agreement in 1974 is considered the same government today. In general, under international law, a state is obligated to uphold agreements made by previous governments. It is too early to tell whether a successor government will uphold those obligations.
This gets a little more complicated by the fact that Syria never recognized Israel. While even parties that don't recognize each other are still obligated by the terms of the agreement, this could affect the legal analysis.
For Israel to abrogate the agreement, it has to adhere to various provisions in the Vienna Convention on the Law of Treaties. Two possible legal avenues that could be pursued are Articles 61 and 62. Article 61 says that if the agreement becomes impossible to enforce it may be ended; Article 62 says that it could be ended by an unforeseen change in circumstances. The bars for both of them to be invoked are quite high, however, and even if Israel would want to invoke them, it would have to give formal notice and wait a time period for a response.
However, Israel has not nullified the agreement. It has said that the moves it has made like moving into the buffer zone are a temporary response to the changed circumstances following the collapse of the Assad regime. They are meant to secure Israel's border since there is no guarantee that any rebel forces could be trusted not to enter the buffer zone and UNDOF couldn't stop them. It is meant to only last until the security situation stabilizes and it becomes clear who will control the Syrian side of the border.
From Israel's perspective, holding onto the peak of Mount Hermon is essential for its self defense in the absence of a reliable UN or other force there.
Moves like these are justified under the necessity of self-defense or maintaining national security. Time is of the essence and Israel cannot afford to wait to secure its border and ensure no dangerous weapons fall into the hands of one of many jihadist groups in Syria. Up until last week, Israel could rely on the Syrian regime to act in certain ways that enforced the disengagement agreement; it can no longer rely on that and needs to make decisions now to protect itself. Even the hundreds of airstrikes can be viewed as proportionate since there has been little or no collateral damage and they were aimed at military targets.
So while it is technically violating the 1974 agreement with a regime that no longer exists, self defense justifies its actions under international law until a new Syrian government can step in and be trusted.