In the spring of 2003, as the statue of Saddam Hussein tumbled down in Baghdad, the leaders in Tehran were worried they were next. They sent Washington an offer to negotiate. The offer addressed Iran's nuclear program and a range of other matters, from Iran's support for Hezbollah and Hamas to the situation in Iraq. But President George W. Bush, having declared Iran a member of the "Axis of Evil" in his State of the Union address in 2002, spurned the offer. At the time, Iran had less than 200 centrifuges for uranium enrichment.

A little more than five years later, George W. Bush left office and the Iranians had over 8,000 such centrifuges. Clearly, ignoring Tehran was not working. Moreover the belated efforts of the Bush-Cheney administration, of which I was a part, did too little and were too late in strengthening the sanctions regime against Iran. In short, there was no policy with regard to Iran in the Bush administration other than, in Dick Cheney's words, "We don't talk to evil."

As a result, by the time President Barack Obama's skillful and methodical diplomacy had made the sanctions regime more international and far more effective, the Iranians had over 19,000 centrifuges. Moreover, the U.S. had amply demonstrated in Iraq that its appetite was far greater than its capabilities. In fact, the U.S. removal of Saddam Hussein and subsequent chaos in Iraq had made Iran the power to be reckoned with in the Persian Gulf. Thus, U.S. leverage over Iran had shrunk considerably, the only real leverage remaining being the new and more comprehensive sanctions Obama's team had achieved.

With that reduced but refurbished leverage, the U.S., China, Russia, Britain, France and Germany opened negotiations with Iran in November 2013. This group, after a year and a half of negotiations, announced an agreement in July 2015. Though most such agreements in the past have been executive branch responsibility, Congress decided to weigh in and to do so substantially, i.e., claiming a right to approve or disapprove the agreement.

Since the political overtones of this congressional move are so clear, it is fair to ask whether their review is principally grounded in a genuine assessment of the best course for U.S. foreign policy, or in pure politics? One might comment: What else is new in Washington? Tax reform, immigration reform, healthcare reform, a $19 trillion debt – these are all serious issues poisoned by politics.

But this issue is different in an important respect: There are potentially deadly repercussions of a U.S. rejection of this agreement.

Rejection means the U.S. is alone. No one else, not even Britain, will follow us. We will be acting entirely unilaterally, without friends or allies (with the exception of Israel's Prime Minister Benjamin Netanyahu). And those who claim that we can simply return to sanctions are hopelessly blind to reality. No one will be with us – absolutely no one. The sanctions will be unilateral and even more feckless than they were before diplomacy achieved comprehensive participation in them. We will be the isolated state.

In our splendid isolation, if we still find Iran's possession of a nuclear weapon "unacceptable," as we have declared, then we have two remaining courses of action available: bombing or invasion. Either of these moves will also be carried out alone because no one will follow us except Israel.

Bombing, despite the self-deluding arguments of men such as John Bolton, the ambassador to the U.N. during the Bush administration, and Sen. Tom Cotton, an Arkansas Republican – arguments highly reminiscent of the "we'll be met in the streets with flowers" sentiments about Iraq in 2003 – will not accomplish the mission. Every military and intelligence analysis I have read predicts U.S. bombing will produce at most a five to seven year delay in Iran's nuclear program, compel Tehran to make a decision to build a bomb as a deterrent against future attacks and drive their work deeply underground to make it impervious to even bunker-buster bombing in the future. This outcome would leave the U.S. with a decision to invade or not – depending on whether or not we still found Iran's possession of a nuclear weapon "unacceptable."

Such an invasion and occupation will require half a million to a million troops, two to three trillion taxpayer dollars, and probably five years to a decade to accomplish even moderate success in subduing all opposition (think Iraq and think a return to conscription).

Iran is four and a half times the size of Iraq, with more than twice as many people, and with a national cohesion unlike any other country in the region. In the event of a U.S. invasion, neither the number of troops in Iran's military nor the size of its military budget would matter. Fifty million or more Iranians would unite to oppose our invading forces. Asymmetric warfare – a fancy name for unconventional warfare such as the U.S. fought in Vietnam – would likely kill 50,000 to 60,000 Americans, wound over 100,000 and make the U.S. decade in Vietnam pale in comparison. Plus, Iraq, Syria, Afghanistan and Pakistan would be boiling, Russia and China would have significant interests at stake and would likely fund and fuel proxies, and the entire region would be in flames. Turkey's remaining in NATO could well be at stake and NATO in general would shudder at its erstwhile leader's strategic recklessness and unreliability.

In addition, the U.S. would expose its troops in Iran to every terrorist group on earth that had the wherewithal to recruit and deploy, from the al-Nusrah Front to Hezbollah (the "A-Team" of terrorists, as the U.S. military rates them), from the Islamic State group, Hamas and Lashkar-e-Taiba, to Jemaah Islamiya, and on and on. Every U.S. service member would have a big, red bulls-eye painted on his or her back. Like a giant magnet, the U.S. occupation force would suck terrorists into its midst. And we must remember the huge proportion of the population in the region that is male and below the age of 35 – an almost endless supply of "soldiers." Of course some of these terrorists would want to kill Iranians as well as American GIs, but that reality simply demonstrates the highly complex nature of such a U.S. undertaking.

I helped with U.S. war-planning in the mid-1980s when U.S. leadership feared the Soviets would roll out of Afghanistan after their invasion in 1979-80 and march straight down to the warm-water ports on the Persian Gulf, particularly Bandar Abbas and Chabahar in Iran. I studied the passes and the compartmentalization of the Zagros Mountains and Iranian geography in general. No place on earth terrain-wise is as bad as Afghanistan, but Iran comes very close. Alexander the Great almost perished there. It is not a place America should go to war unless near-annihilation of Iran's population and industry is the objective and the full panoply of U.S. power is unleashed. That is simply not going to happen, despite the call of some Luddites for it to happen.

"Don't take counsel of your fears" is an age-old aphorism. But like so many such soothsayer sayings, it often masks its opposite and just-as-wise counsel: "Don't overestimate your capabilities." The U.S. overestimated its capabilities to great damage in Vietnam and Iraq. We must not repeat that huge mistake with Iran.