A nuclear accord between Iran and the five permanent members of the United Nations Security Council, plus Germany, is almost certain. In this context, of particular importance will be an agreement reached between Tehran and Washington.

The Obama administration has embarked on an intense campaign to reassure its friends and allies. Nevertheless, an accord will likely herald a new order in the Middle East, in which assumptions will change. This was the gist of what Gen. John Allen, the U.S. envoy in the coalition to fight ISIS, said in Doha, Qatar, last week in a keynote address before the U.S.-Islamic World Forum of the Brookings Institution.

The old assumptions have already been eroded by the rise of ISIS. Emerging alliances to counter this common threat will be reflected in Lebanon, a fault line between the United States and Iran. That is why the country must adapt to the political ramifications of a deal reached between the West and Iran.

The sharp divisions between the March 8 and March 14 coalitions, which have dominated the domestic political scene, may become less pronounced with a thaw in U.S.-Iranian relations. A nuclear accord will allow the U.S. and Iran to consolidate their influence. In the face of a rising ISIS, competition and opposition may give way to synergy.

Already, there are signs of mutual tolerance on the ground, echoing the rapprochement between U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif. Anti-American posters have been quietly removed from Shiite neighborhoods in Beirut’s suburbs. Sayyed Hassan Nasrallah’s attacks on the U.S. have been muted and confined to what he has described as “Saudi-American aggression” in Yemen, with no reference to the CIA’s being implicated in the assassination of Imad Mughnieh in Damascus.

A USA Day at the Lebanese-American University in West Beirut, with huge U.S. and Lebanese flags on display, was well received, even by the Hezbollah student body. There is tacit Hezbollah acceptance and encouragement of U.S. aid to the Lebanese Army as well as of reconnaissance missions carried out by U.S. special forces, using unmanned aerial vehicles, over the northeastern Bekaa Valley. These flights are taking place alongside those of Iranian-controlled unmanned aerial vehicles.

The U.S., in turn, is accepting of Hezbollah’s presence in the Lebanese government as well as its coordination with the Lebanese Army in confronting radical Islamist groups in the Qalamoun area. Hezbollah’s massive involvement in Syria, though denounced by Washington, has not changed U.S. policy.

Lebanese political parties may not need to be with America against Iran, or vice versa. In fact, Lebanese on both sides of the political spectrum may feel more at ease to engage with one another. The Phalange party is in a dialogue with Hezbollah. A Lebanese Forces-Hezbollah dialogue, unthinkable a year ago, may soon gather pace. Hezbollah’s reliance on Michel Aoun may decline in relative terms, in the same way as the March 14 Christians’ reliance on Saad Hariri, their principal Sunni ally.

Predictably, sectarian tensions may be exacerbated by a nuclear agreement. The U.S. has embarked on a campaign to reassure it’s Gulf Cooperation Council allies by reaffirming its strategic commitment to their security. Indeed, the U.S. air base in Qatar, the naval base in Bahrain, and the military base in Kuwait are all proof of this commitment. Iran is incapable of and unwilling to challenge this reality – or for that matter to challenge the U.S. position in Yemen.

The situation in the Levant is another matter. Zarif, in a landmark article in Foreign Affairs, indicated that Iran’s primary interest, apart from friendly relations with the United States, was in the Levant. The U.S. is leading a coalition against ISIS that has stretched to the Levant, but the Obama administration is unwilling to put boots on the ground. ISIS is a regional problem with global implications, according to John Allen. Therefore, the war against ISIS must be fought by regional forces, with logistical, intelligence and tactical support from abroad. Iran and its allies will bear the brunt of the fighting against ISIS in Syria and Iraq. Iranian hegemony in the Levant will not come cheap, if it ever comes.

In the absence of a credible Iraqi army, the Shiite Popular Mobilization forces are the spearhead of the fight against ISIS in Iraq. In Syria the Assad regime is capitulating on several fronts in the north and the south, necessitating a rapid and frequent infusion of foreign Shiite fighters. In Lebanon, Hezbollah’s offensive in Qalamoun is, partly, a defensive maneuver to seal the Lebanese-Syrian border and guarantee the continuity of the Damascus-Latakia corridor. In time, Iran may find it reasonable to defend its Shiite assets and leave Sunni areas in Iraq and Syria to ISIS, which some in Iran consider to be a Sunni problem for Saudi Arabia And Turkey to resolve.

Such a scenario could leave Lebanon in a precarious position. Rising sectarian tensions as well as increasing mistrust and inflammatory rhetoric have already neutered the dialogue between the Future Movement and Hezbollah. The vacancy in the presidency as well as an increasingly dysfunctional government seriously jeopardize an important sectarian safety net. The Lebanese Army remains steadfast as the most important and perhaps last such safety net. That is why, while the Army is coming under increasing pressure to participate in Hezbollah’s offensive in Qalamoun, it has wisely refrained.

While Lebanon weathered the sectarian tensions of 2014, after the fighting in Arsal, it seems less prepared for the aftermath of a nuclear accord. A perceived lack of Christian representation does not constitute a clear and present danger to the country. A new president, like the Army commander, should primarily act as a sectarian buffer between Sunnis and Shiites. Dialogue between the communities must continue and expand.

Engaging moderate Sunni forces in Lebanon committed to the Constitution and security agencies should remain the cornerstone of U.S. policy in confronting violent extremism. This is all the truer in that Sunnis will perceive a nuclear deal between Washington and Tehran as a U.S. tilt toward Iran.

A word of warning. Those contemplating a change in the Taif accord to adapt to the new reality of friendlier U.S.-Iran ties may want to think twice. The ensuing disagreements could mean that Lebanon, too, could go in the direction of a failed state, as it becomes consumed by internal discord. And minorities have not fared well in failed states.