What Leverage Does a Nuke Earn Iran?

Kroenig, via Yglesias:

The United States’ global power-projection capability provides Washington with a significant strategic advantage: It can protect, or threaten, Iran and any other country on the planet. An Iranian nuclear weapon, however, would greatly reduce the latitude of its armed forces in the Middle East. If the United States planned a military operation in the region, for example, and a nuclear-armed Iran objected that the operation threatened its vital interests, any U.S. president would be forced to rethink his decision.

Really? Why? I can see how a US President might be forced to rethink a decision that would lead inevitably to the destruction of the Iranian state; in such a scenario, we might plausibly expect that there would be at least a chance that the Iranians would use a nuclear weapon, although of course the first step of any such intervention would be extremely heavy bombing attacks on all known or suspected Iranian nuclear weapon sites. But for anything short of actually conquering Iran and overthrowing its government, do nuclear weapons give Iran any additional leverage? Does anyone think that Iran would have used a nuke in protest of the US invasion of Iraq, or Israel's offensive against Hezbollah? Or the establishment of US bases on Iran's borders? Under what circumstances, exactly, would an Iranian nuclear weapon actual force the United States to reconsider some action it wished to take?

I understand the stability-instability paradox, the idea that the strategic stability produced by nuclear weapons can produce low level instability. If that's the argument Kroenig is making (having a nuke will allow Iran more latitude in mucking about in Iraq or Lebanon or Gaza) then it should be made with greater clarity (and perhaps it is in the book). Even here, however, I doubt that the stability-instability paradox really holds; US conventional capabilities are so advanced that I very much doubt that any Iranian nuclear capability could survive a US conventional first strike.

Another way to think of this is to look to the example of Pakistan. The Pakistani nuclear deterrent hasn't prevented the United States from overthrowing Pakistan's client in Afghanistan, continuing the fight against that client for nine years (as the fight destabilized Pakistan's border regions), and even launching a long campaign of attacks within Pakistan's borders. It's almost enough to make one doubt that nuclear weapons actually provide any serious leverage in ordinary diplomatic and military disputes.