The chickens of Israel’s botched 2006 war against Hezbollah in Lebanon are coming home to roost. Only now are we beginning to see the full consequences of the campaign’s failure.

It would be an exaggeration to say that Hezbollah’s taking over the streets of Beirut these past few days has been an armed challenge to Lebanon’s government. Lebanon does not have a government. It has not had one since its civil war of the 1970s and 1980s. It has a prime minister, a cabinet, and a parliament, none of which can, by any stretch of the imagination, be said to govern. It used to have a president, an office that has been empty for many months with no noticeable effect on anything. And it also has a small and poorly equipped army that, although it has on rare occasions been known to fight, has mostly preferred to cede the field to others, as it did in recent days to Hezbollah.

Lebanon has been governed, ever since all central authority in it fell apart several decades ago, by shifting coalitions of militias and warlords belonging to the country’s different ethnic and religious groups, and by foreign countries allied with them, mainly Syria, Iran, and Israel until its withdrawal from the Lebanese south in the year 2000.

For a long while Hezbollah has been one of these factions, and the military show of strength it now has put on is first and foremost a challenge to the others.

It is a dare to them to step up and fight, and the fact that none of them has chosen to do so — neither the Sunnis, nor the Christians, nor the Druze, nor the Palestinians, nor Hezbollah’s erstwhile rivals among Lebanon’s Shiites, the now virtually defunct Amal party — is a sign that the old balance of power in Lebanon has been broken.

Hezbollah now has become so much better armed, trained, organized, and motivated than any other armed group in Lebanon that there is no one left to resist it. It may not be strong enough to seize control of the entire country the way its ally, Hamas, seized control of Gaza, but it is certainly strong enough to keep expanding its domain of a state-within-a-state without fear of being checked by counterforce. In a country of militias, it has become the one super militia.

Nor does Hezbollah, supported by Syria and Iran, need to fear any force outside of Lebanon. After Israel’s poor and costly showing of the summer before last, it is not about to try again; what it could not accomplish under relatively favorable circumstances two years ago cannot be accomplished under less favorable circumstances today.

As for Europe and America, neither has the ability nor the will to intervene on the side of Hezbollah’s rivals. The U.S. is overextended militarily and is not about to get embroiled in another Arab country. The new European-led UNIFIL, the supposedly revamped United Nations Force In Lebanon that emerged from the 2006 war and from Security Council Resolution 1701 that ended it, has proven to be as much of a fiasco as was the old UNIFIL that preceded it.

Who even remembers today that 1701, which at the time was brandished by Israel as a diplomatic victory that justified its invasion of Lebanon, called for such measures as a Lebanese army deployment in the south of Lebanon to replace Hezbollah and to re-establish the authority of the Beirut government in the area; the disarmament of all armed groups in Lebanon so that “there will be no weapons or authority in Lebanon other than that of the Lebanese state;” the securing by the government of Lebanon of all “borders and other entry points to prevent the entry into Lebanon … of arms or related materiel;” and the barring of “the sale or supply to any entity or individual in Lebanon of arms and related materiel of all types, including weapons and ammunition”?

Every one of these provisions has been flouted by Hezbollah, which has in the past two years received a steady flow of weapons from Syria and Iran that has made it far better equipped and more dangerous than it was before the 2006 war. And every one of these floutings has found UNIFIL looking eagerly the other way. There has not been one single case of UNIFIL using force or the threat of force to make Hezbollah comply with 1701’s principles.

For Israel, the lesson, if there was any need for it, is clear: When it comes to relations with its Arab neighbors, it must never, but never, rely on international guarantees of any sort. They are worthless. Yes, they will sometimes be honored by these neighbors, as Egypt has honored its 1979 peace treaty in Sinai and Syria has honored its 1974 disengagement agreement on the Golan Heights, but this is only because they fear Israel’s reaction, and not the international community’s, if they break their commitments.

In Lebanon, no such fear exists. Hezbollah knows Israel will not reinvade, just as it knows UNIFIL will not lift a finger to carry out the United Nations mandate that it was entrusted with.

This is a lesson to be especially taken to heart in Israel’s dealings with the Palestinians. No peace treaty with the Palestinian Authority that depends in any way on international observers, supervisors, or enforcement should ever be considered, much less accepted.

No one has the power to make the Arabs keep their promises to Israel but Israel itself. Hezbollah’s unimpeded march to dominance in Lebanon is a sad example of what can happen when this rule is ignored.

Mr. Halkin is a contributing editor of The New York Sun.