Afghan president Hamid Karzai, U.S. president Obama and Pakistani president Asif Ali Zardari.The explosive reaction to an online film denigrating the Prophet Muhammad showed how readily a single provocation can inflame so many in the Islamic world. But these demonstrations and the violence that accompanied them will seem only a modest dress rehearsal for what can be expected following an Israeli attack on Iran.

If Iran is bombed, the response from the street will be far more intense and widespread, and the position of Muslim states will be less ambiguous as to where their sympathies lie. No less clear is that the United States will serve as the prime target of the anger—whether or not it bears any responsibility. And nowhere is the fallout of an attack on Iran’s nuclear facilities more likely to upend U.S. policy and put in harm’s way larger numbers of Americans than in Afghanistan and Pakistan.

For many years, Pakistanis and Afghans, beset with their own national challenges and obsessions, were only marginally attentive to events in the Middle East. While they were always sympathetic to the Palestinian cause, cultural and institutional legacies of colonialism and military alliances drew Pakistan’s educated public toward closer identification with the West. In Afghanistan, the heavy aid dependency on the Soviet Union and the United States through the 1970s largely removed the country from the influence of other Muslim states. Pakistanis and Afghans also found their richer Arab and Iranian coreligionists often condescending and standoffish.

But three decades of jihads fought by Afghans and Pakistanis drew them closer to Arab governments as well as to private individuals and organizations that injected financial support. These sources also helped nurture a pan-Islamic consciousness and stoked anti-Western feelings. Rather than 9/11 aligning these countries with the West in recognition of the common threats posed by international terrorism, the West’s war on terror became widely perceived as an attack on all Muslims. In present overheated political climates in Pakistan and Afghanistan, an air strike on Iran is sure to evoke a popular reaction that threatens American assets and intimidates those in Kabul and Islamabad who are responsible for defending them.

An Israeli attack on Iran would heavily impact the tenuous security, economic and political transitions underway in Afghanistan. Demonstrations and random violence would force U.S. and allied troops to hunker down and encourage the Taliban to step up their insurgency. If the Afghan president and his government joined the chorus casting blame on the United States, the still inchoate strategic agreement with Kabul, which had been expected to provide for some twenty thousand U.S. troops to continue training Afghan soldiers and police, could very well be aborted.

Congress and the American public would increase pressure to beat a hasty retreat from Afghanistan. Most aid programs would be put on hold as U.S. and other foreign aid workers scurry to the relative safety of Kabul or leave the country entirely. Critical financial assistance pledged by Afghanistan’s benefactor nations likely would be in jeopardy. In the new political climate, whatever hopes Washington might have had that the Taliban leadership could be induced to reach a political settlement would vanish.

In neighboring Pakistan, popular outrage at the bombing of targets in Iran could be even stronger. Events could follow those of 1979, when the U.S. embassy and other facilities were burned on a mere rumor that the United States was implicated in a radical Islamic group’s attack on the Grand Mosque in Mecca. Better organized than in Afghanistan, jihadi and other extremist groups can be expected to capitalize on any new discord with the United States as a way of furthering their domestic agendas. Energized, these groups and the Pakistani Taliban would gain new credibility with the public as they spearhead anti-American demonstrations.

Although the Islamabad government can be expected to reactively condemn Israel and the United States, the People’s Party, which is generally viewed as more sympathetic to the West, will have difficulty holding its ruling coalition together. In general, more progressive elements in Pakistan are likely to seek cover by denouncing Israel while deploring reactive domestic violence. The army will act to restore order but only after allowing the crowds to vent their anger.

With political forces long disposed to end cooperation with the United States, NATO supply routes to Afghanistan may again be blocked, and critical air rights over Pakistan may also be withdrawn. These actions are tailor-made to prompt a complete aid cutoff to Pakistan and virtually ensure a complete rupture of relations—with all its consequences for Washington’s ability to keep tabs on Pakistan’s nuclear arsenal and international terrorist organizations.

An attack on Iran would set in motion the further radicalization of Muslim populations in Afghanistan and Pakistan, which might initially unite them but will soon foster instability and exacerbate old conflicts. Any popular support for a religion-driven militancy promises less conciliatory domestic and foreign policies in either country. The intolerance that extremist groups will generate reduces the likelihood of an inclusive approach to avert civil war in Afghanistan and rouses demands for ethnic separatism in Pakistan.

The ramifications of an Israeli strike on Iran could extend beyond the region. A strengthened jihadi narrative in Pakistan, together with an Indian government spooked by its own aroused large Muslim community, could push the two countries toward armed confrontation again. Disengagement in the aftermath of an Israel strike would also mark a watershed for the U.S. role in South and Southwest Asia, leaving a vacuum to be filled by China, Russia, Saudi Arabia and Iran that ensures increased external meddling in this already volatile region.

As Washington envisions these outcomes, it must factor them into how closely it wishes to align its policies with Israel in reining in Iran’s nuclear ambitions. If the United States is going to abet Israel’s military actions, it must carefully weigh the potential threat posed by a nuclear-capable Iran against the predictable damage to U.S. strategic interests—not just in Afghanistan and Pakistan but also on a global scale.

While Israel’s decision makers may choose to define their country’s vital interests more narrowly, they must be reminded by Washington that in the long term Israel’s security still depends on there being a strong American presence around the world. If by precipitous military action Israel seriously undermines U.S. influence and power, Israel’s own security also will be sharply diminished.

Marvin Weinbaum is the director of the Middle East Institute’s Center for Pakistan Studies and a former intelligence analyst for Pakistan and Afghanistan in the U.S. Department of State.