When Hezbollah made its fateful decision to intervene militarily in the Syrian civil war, it was only a matter of time before the war would follow them back home and ignite a fire in Lebanon. This month three car bombs went off in Lebanon, killing scores and injuring hundreds.

The first bomb, which exploded in Dahiyeh, Hezbollah's stronghold in southern Beirut, killed 27 and injured many more. No-one claimed responsibility for the blast, but few Lebanese doubted that it was a message from supporters of the Al-Qaeda-like wing of the anti-Assad Syrian rebels. A few days later, two car bombs exploded outside of mosques in the northern Lebanese city of Tripoli, a Salafi stronghold. The twin blasts killed 47 people and injured hundreds. Many Lebanese Salafis in the north are Sunni supporters of the Syrian rebels.

Hezbollah went to war in Syria only to encounter, among other foes, Al Qaeda. Salafist-Jihadists apparently brought the war home to them as well. This series of tit-for-tat bombings has created the most violent and volatile dynamic in Lebanon since the end of the civil war. The Syrian war, predictably enough, followed Hezbollah right back to its home territory.There was, by the way, a muted but palpable sense of Schadenfreude in Israel at seeing Hezbollah and Al Qaeda go to war in Lebanon.

When the Syrian conflict first broke out, it quickly spread to the northern parts of Lebanon around Tripoli, which contain atypically conservative Lebanese Salafi Sunni communities, alongside Alawite ones. With their comrades and coreligionists at each other's throat just across the border, these forces could not resist combating each other in northern Lebanon.

But the major political forces in Lebanon decided to try to quarantine the spill-over effect to that northern area. Lebanese politics for more than a decade have been characterized by an equilibrium of unstable elements. No one wants to start a generalized war in Lebanon because no one can have any confidence of prevailing, and everyone stands an excellent chance of losing more than they gain.

Additionally, modern Lebanese history has made one thing inescapably clear: any single group —internal or external—which attempts to assert hegemonic control over the entire country quickly faces the united opposition of virtually all other forces, and are eventually pushed back into their home redoubts. This happened more than once to right-wing Maronite forces, a left-wing/Palestinian alliance, Israel, and, eventually, even Syria. So all the parties that are fundamental beneficiaries of the status quo in their own areas have very strong disincentives from trying to ignite a conflict that is unlikely to improve their position.

But Hezbollah, knowing full well the risks of its intervention in Syria, nonetheless believes that the survival of the Bashar Assad regime is simply an existential necessity. Syria is the direct lifeline between Hezbollah and its patrons in Tehran. Without that contiguous link, and the support of the most powerful external influence in Lebanon—i.e., Syria —the existence of Hezbollah's quasi-sub-state in its current form, with its independent foreign and military policy, would be placed in serious jeopardy.

Hezbollah is not going to go away. Like all Lebanese political parties it has, essentially, a Janus-faced dual-existence. On the one hand, they represent their local constituencies. In the case of Hezbollah, it is the biggest Lebanese sectarian community—the Shiites, among whom they are the unrivaled leaders. (There is no majority community in Lebanon.) But, like all other Lebanese parties, it also represents a foreign patron, in this case Iran.

In its loyalty to its external patron, Hezbollah is essentially the creature of the Iranian Revolutionary Guard. And while it was possible to speculate over the past decade about whether it had actually developed an independent political character, the Syrian war has put this question to rest altogether. Hezbollah remains entirely joined at the hip with its Iranian patrons and both are committed, existentially, to the survival of the Assad dictatorship at all costs.

So, Hezbollah took the drastic, reckless and unspeakably irresponsible—but also probably, from its own point of view, indispensable—step of dispatching many of its elite units to fight alongside Syrian forces in defense of some of the most strategically significant areas being contested.

For some time, inevitably, there has been some spillover effect from the Syrian war into Lebanon. It contributed to a condition of paralysis in Lebanese politics over the past five months since the resignation of Prime Minister Najib Mikati, ostensibly over a disagreement about scheduling new elections. The reality was the split within the government reflected political disagreements over the war in Syria, Hezbollah's intervention, and competition between Saudi and Iranian clients in Lebanese politics. But still the violent spillover was mainly quarantined in the north, since one of the few things most Lebanese can agree on is that another civil war is not in their interests.

Al Qaeda and similar Salafi-Jihadi organizations, while present in the north in small numbers, are decidedly unpopular among Lebanese Sunnis. Indeed, the very name Al Qaeda is so anathema in the Levant in general that it always has to rebrand itself there: "Jabhat an-Nusra" in Syria and other similar marketing ploys in Lebanon and among the Palestinians. "Al Qaeda," presented as such, would be rejected out of hand as a gang of maniacal barbarians. Unfortunately, the selfsame ideas and behavior nonetheless find a constituency even when the traditional brand is tarnished beyond repair.

The effort to quarantine the spillover from the Syrian civil war in the north of Lebanon is breaking down in a bloody and dramatic manner. Hezbollah's intervention in Syria will continue to haunt them deep into Lebanon, surely in an ever increasing and intensified manner. They have placed everything at risk, and they know it. And, yet again, they have acted recklessly at the expense of the rest of Lebanon, and at the behest of their Iranian patrons. Containment so far has had some relative success: Lebanon is not in anything like the condition Syria faces. But, for how long?

The damage thus far is significant. First, the post-1998 commitment of regional parties not to use Lebanon as a proxy battlefield, as they have traditionally done in the past, is breaking down. Second, because Hezbollah has committed itself so firmly to the outcome in Syria—and because Syria is the most important external power in Lebanon—the outcome of the Syrian war will have a profound impact on the Lebanese political equation. Most Lebanese parties are genuinely trying to avoid an epidemic of sectarianism. But under the circumstances, they seem unable to stop its inexorable metastasis.

There can be little doubt that the Lebanese state is fracturing more than ever, both in terms of its institutions and political structures, and in the delicate equilibrium between its political factions. Hezbollah's intervention in Syria is the primary catalyst, but hardly the only cause. Just as the effort to quarantine spillover violence from the Syrian war in the Lebanese north is failing, so, too, are prospects of ever-greater sectarian violence throughout the country growing, as is the dim but plausible prospect of yet another round of civil conflict.

The irony is this: Syria is fracturing in a manner eerily reminiscent of Lebanon's sectarian civil war of 1975-89. Since then, Lebanon has been a country largely at peace, but fundamentally disintegrated: It is a collection of sectarian and ethnic enclaves (with greater or lesser degrees of autonomy) loosely held together by a very weak and increasingly dysfunctional Beirut-based government which, in many crucial ways, does not even make the pretense of performing basic state functions.

It's not exactly a failed state, but Lebanon is a fractured, dysfunctional and disintegrated republic. Since 1989, this arrangement has provided a workable degree of stability, but one based on an equilibrium of unstable elements. The long-term viability of this arrangement has always been questionable at best, and is currently undergoing its greatest test since the 1989 Taif Agreement brought an end to the civil war.

The long-term outcome in Syria is impossible to predict: there are far too many variables and imponderables. But it seems extremely unlikely that the modern, centralized, Damascus-ruled, integrated nation-state Syria has been since independence in the 1940s will survive the current war.

A not-particularly pessimistic view might hold that a best (or least-worst) case scenario for Syria in the medium-term is a Lebanon-style future: a country that is theoretically integral but in fact is profoundly fragmented, and ruled locally by various sectarian and ethnic power brokers, with greater or lesser autonomy of action, and all held together by a very weak central government in Damascus whose writ does not run in most of the nominal nation.

The irony of ironies begins with the fact that while this "least-worst" scenario for Syria might mirror what has actually been in place in Lebanon for the past two decades. And the irony is even further compounded since this very arrangement in Lebanon is now placed in mortal peril by forces that are pushing its northern neighbor in a very similar direction. So the question is, can Lebanon survive the "Lebanonization" of Syria without another abominable round of bloodletting? Can Syria become a "Lebanon" without wrecking that fragile model in Lebanon itself?

It is essential to emphasize that these questions are entirely based on avoiding more dystopian scenarios in both Lebanon and Syria, which scarcely bear dwelling upon. Thankfully, they are less likely then the grim but tolerable Lebanese—and perhaps emerging Syrian—stalemate and equilibrium of unstable forces in fundamentally fragmented but not failed states. Believe it or not, there is nothing particularly pessimistic about that evaluation. Try talking to a real pessimist.