Ever since key Iraqi and Syrian cities fell to ISIS last month, the administration has been scrambling to adjust its tactics.

Rather than revamp a failed strategy, U.S. officials now appear ready to rely (at least tacitly) on Iran to help roll back the jihadis. This is especially true in Iraq, where Iranian-backed Shiite militias have proven more effective in fighting ISIS than the Iraqi army has.

Previously wary of these Shiite militias - lest they inflame sectarian tensions and push more Sunni Iraqis into the ISIS camp - U.S. officials have now dropped objections to their playing a major role.

This is a mistake. True, the ayatollahs are bitter enemies of the Islamic State, but their goals in Iraq differ greatly from those of the United States. The enemy of my enemy isn't necessarily an ally, even indirectly.

That's because Tehran has little strategic interest in wiping out the Islamic State.

I'll get to why that's so in a moment. First, it's necessary to revisit why the U.S. strategy has failed.

Until late last month, that strategy revolved around efforts to retrain divisions of the Iraqi army. Much of that army had collapsed a year ago when ISIS took over Mosul, and surrounding Nineveh province, which is largely inhabited by Sunnis.

The initial step of the strategy was to push ISIS out of Anbar province in the west of Iraq, home to large Sunni tribes. But the Iraqi army was far from ready to liberate Anbar, let alone Mosul - and won't be ready to take on ISIS anytime soon.

Yet the Shiite-led government in Baghdad, backed by Iran, was unwilling to help Sunni tribes in Anbar that wanted to fight ISIS. Around 2,500 Anbar Sunnis got only a smatter of training and a pittance of light arms, a far cry from the mid-2000s, when Washington armed and backed Sunni tribes who drove al-Qaeda out of Anbar.

U.S. officials, for their part, chose not to arm Sunni tribesmen directly or pressure Baghdad strongly to permit those tribes to set up national guard units. Washington maintained the fiction that some Sunni tribesmen could be integrated with Shiite militias that weren't linked with Iran - a mismatch that never jelled.

Meantime, the Iraqi national army presence in the province was too thin to fight the heavily armed ISIS with its convoys of truck bombs. Without U.S. spotters on the ground, American air strikes couldn't help.

So Ramadi fell, and the jerry-rigged U.S. plan for liberating Anbar collapsed.

Iran, on the other hand, made certain that its Shiite proxy militias in Baghdad were better armed than the regular Iraqi armed forces. It sent Gen. Qasim Soleimani, a powerful Revolutionary Guard commander, to supervise their operations in Iraq.

In the wake of the Ramadi debacle, Washington has dropped its opposition to the use of Iranian-backed Shiite militias in liberating Sunni territory. The special U.S. envoy on fighting ISIS, retired Gen. John Allen, said last week that the militias have an important role to play in Anbar province so long as they "take command from the central authority." He meant so long as they follow Baghdad's orders, not Tehran's.

"And they are now doing that," Allen added.

Would that this were so, but there is little visible evidence to prove it.

The fear in Anbar is that Shiite militias will attack Sunni civilians and drive them out. That's what happened around Baghdad. These fears were inflamed when the militias - now supposedly a part of the Iraqi army - named their forthcoming Anbar campaign Lubaik Ya Hussein. This loosely means "the Shiites are coming" to fight in the name of their religious icon, Hussein, grandson of the prophet Muhammad.

The militias later changed their campaign slogan. But these Iranian proxy forces clearly believe they are taking over the Iraqi army, rather than vice versa.

Which brings us to the issue of Tehran's goals in fighting ISIS in Iraq.

During my recent visit to Iraq, Kurdish, Iraqi military, and Sunni tribal officials all told me the same story: In Anbar, the ayatollahs' aim is not primarily to destroy ISIS. Rather, they seek to consolidate a land bridge westward to Syria, whose Shiite leader, Bashar al-Assad, they are supporting. And they want to protect Baghdad suburbs and Shiite holy sites southeast of Baghdad that abut Anbar.

However, Iran and its proxies have little interest in whether ISIS continues to rule over Sunnis in other parts of Anbar - or in Mosul. Those Sunni areas do not abut Iran, or Baghdad, or the Shiite provinces of southern Iraq. It isn't worth sacrificing Shiite lives to liberate them.

In other words, if ISIS keeps its caliphate in Mosul, and keeps killing Sunnis, it won't upset Tehran.

Yet the Iraqi government - dominated by Shiite political parties - rejects the idea of helping Sunnis in Anbar and Mosul form strong national guard units that could drive ISIS out, with the help of U.S. air strikes.

Without a shift in U.S. strategy to help Sunnis directly, ISIS will remain in Iraq indefinitely, as its tentacles spread elsewhere. Iraqi Sunnis will be doomed to live under ISIS - or risk being driven from their homes by Shiite militias. Under such conditions, it is no surprise that many Sunni tribal leaders are declaring their allegiance to ISIS.

When President Obama meets with Iraqi President Haidar al-Abadi in Paris on Monday, he should insist that Baghdad aid Sunnis directly. If the well-intentioned Abadi is too weak to do so, then the United States must - or consign the fight against ISIS to failure.

Relying (even tacitly) on Iran's proxy militias to take out the ISIS caliphate is nothing but a mirage.