There is no military solution to the war in Iraq. Our troops can help suppress the violence, but they cannot solve its root causes. And all the troops in the world won't be able to force Shia, Sunni, and Kurd to sit down at a table, resolve their differences, and forge a lasting peace. In fact, adding more troops will only push this political settlement further and further into the future, as it tells the Iraqis that no matter how much of a mess they make, the American military will always be there to clean it up.
You bomb ISIL. You're not trying to bomb innocent people. And that requires intelligence and confidence in our military to be able to develop the kinds of targets that we need. We're already doing Special Forces, who are going to help us gather that intelligence and help advise and assist and train local forces so that they can go after ISIL in areas like Raqqah and Mosul.
Any brief military advantage the USA might gain with nuclear weapons would be offset by political and psychological losses and damage to American prestige. The United States might even touch off a worldwide armaments race.
What are the conditions that make for the superiority of an army? Its internal organization, military habits in officers and men, the confidence of each in themselves; that is to say, bravery, patience, and all that is contained in the idea of moral means.