A great problem in United States foreign policy is the threat of Iran obtaining a nuclear weapon. There are three general solutions to this problem - 1) containment, 2) prevention, or 3) diplomacy. In other words - 1) living in a world with a nuclear Iran, 2) taking the risks of war to delay Iranian progress for some years, or 3) finding an imperfect deal that allows the international community to monitor and react to Iran's potential cheating.
The Obama administration prefers and currently attempts a diplomatic solution. It will likely fail, but the efforts should be commended nevertheless. The administration's official policy is to prevent Iran from obtaining a nuclear weapon by any means necessary. This implies by war if necessary, but are they bluffing?
The answer to this question determines the result if diplomacy fails. If the administration is willing to go to war over Iran's nuclear program, then failure to reach a deal means war. If they calculate war to be too risky, then diplomatic failure will lead to a nuclear Iran. Only those to whom the president has revealed his true intentions know the answer to this question. However, neither outcome is ideal.
War, even limited bombing, is expensive. Military operations are not only expensive in dollars but potentially in lives. War is also risky. Iran has a variety of means to retaliate. They can launch strikes on Israel, mine the Strait of Hormuz limiting the flow of oil from Gulf countries, commit cyber attacks, attempt terrorist attacks, and kill and destroy our military's troops, equipment, and supplies.
Containment has its own set of risks. A nuclear Iran may act even more boldly by further funding terrorist organizations and disrupting U.S. goals in the region. A nuclear Iran may incite an arms race that leads to more Middle Eastern countries obtaining nuclear weapons. Such a situation means the U.S. cannot stand by if these authoritarian regimes collapse to more democratic forces because in the chaos terrorists may capture the weapons of mass destruction. A nuclear Iran gives the U.S. an incentive to support the theocratic regime rather than allow their weapons to fall into the hands of terrorists if the government were to lose power.
Whatever Obama's plan B is, citizens, politicians, and pundits should only act constructively to support the current diplomatic efforts. Any diplomatic solution will be imperfect, but it is the only plan A we got.