TEL AVIV - During World War II, Winston Churchill famously drew a distinction between 'the end of the beginning' and 'the beginning of the end.' That distinction is equally applicable to the unfolding Syrian crisis. Recent events - the growing number of high-level defections from the regime's leadership, the killing of three of President Bashar al-Assad's most senior officials in a bomb attack, and the rebellion's spread into Damascus itself - suggest that, after a long period of gradual decline, the Assad regime is now approaching collapse or implosion.

The Syrian crisis has been raging since March 2011. After several months of mostly quiet demonstrations and brutal suppression, a pattern emerged. The political opposition - divided and ineffectual - was reinforced by a hybrid and loose military wing operating under the banner of 'The Free Syrian Army,' and by hundreds of jihadis who entered Syria through porous borders and began to launch both military action and terrorist activity. The opposition, political and military, could not topple the regime, and the regime could not quash the opposition.

The regime benefited from the active support of the Alawite community and the passive stance of other minorities, as well as of the bourgeoisie in Damascus and Aleppo, whose members feared the regime's fall and its replacement by Islamists or other radical groups. Externally, Russia and Iran acted as the regime's main supporters, while Western countries, Turkey, and Arab states like Saudi Arabia and Qatar extended limited support to different opposition groups.

In military terms, the battle was a draw, but the regime kept losing political ground. The central government's machinery seemed intact, and life in Damascus and Aleppo retained a semblance of normalcy, but the regime lost control over increasingly large parts of the country. Conditions were exacerbated by a sectarian civil war between Alawites and Sunnis, which culminated in several atrocious massacres.

The worst inter-communal fighting took place in the plains to the east of the Alawites' mountain strongholds, raising the suspicion that Alawites were preparing for a retreat to their native region in the event of the regime's collapse and were trying to expand the area under their control.

This pattern of steady erosion has now ended, with senior military and other officials joining the opposition in increasing numbers. Most notably, the Tlas brothers - Firas, a businessman, and Manaf, a general and personal friend of Assad - were the first members of the regime's inner core to defect. These defections have weakened the regime, reinforced the opposition, and sent a message of inevitable collapse.

So, too, has the opposition's major achievement in striking at the heart of the security establishment, killing three of Bashar's most important aides: his brother-in-law Asef Shawkat, former Defence Minister Hasan Turkmani, and his successor, Daoud Rajha.

At the same time, serious fighting has now spread into the heart of Damascus. Significantly, whereas the regime previously sought to downplay the challenge posed by the opposition, Syrian state television has covered the fighting in Damascus extensively. The message, it seems, is that a moment of decision is approaching.

It is still too early to predict the regime's imminent collapse. It has been knocked back on its heels, but it is still standing, and responded swiftly to the assassination of three major figures, losing no time in appointing a new defence minister. Most of the forces that have kept the regime in place for the last 16 months are still there, the opposition remains divided, and the US and its Western allies are still shy of exerting full pressure on Assad's government.

But the end is approaching, and serious thought must be given to several inherent dangers in the Syrian situation. In the absence of an effective, well organised, and internationally recognised opposition, the regime's downfall could be followed by anarchy, all-out sectarian civil war, secessionist movements, and de facto partition.

Large numbers of refugees could flee into neighbouring states, which could be drawn into the conflict. Chaos and fighting could easily spill over into such weak neighbouring states as Iraq and Lebanon. Turkey, always fearful of repercussions among its own Kurdish population, is certainly a major candidate to intervene.

Another looming threat is a scenario in which the Assad regime's stockpiles of missiles and chemical weapons fall into - or are actually transferred to - the wrong hands. Israel has maintained a cautious stance thus far, but has indicated that it will not remain passive if such weapons end up with Hezbollah. Nor can the prospect be ruled out that the regime would seek to depart in a blaze of glory with a final desperate act.

Such risks demand far more effective and coordinated international action to prevent Syria's internal struggle from becoming a grave regional and international crisis. Time is growing short.

Itamar Rabinovich, a former ambassador of Israel to the United States (1993-1996), is currently based at Tel Aviv University, New York University, and the Brookings Institution.