Here's some factors to consider:
President Obama offered to leave a contingent force in Iraq before we ended the War. The Iraqi Government chose by their own volition to not host American troops. America respected the will of the Iraqi government.
ISIS and the other militant groups are no rag-tag group like our propaganda displays them as. These are elite and hardened warriors, some of the most militarily hardened in the entire world. You do not have to like them, but do not underestimate them.
This war is not a one-sided affair. You have multiple factions some intra-Iraq, some international.
Iraq borders Syria, where the currently most bloody war at this moment in time is occuring.
With all that said from the news I seen militants have seized Mosul (extremely important city) and also Tikrit. An estimated 500,000 people have been forced to flee and become refugees. It is now quite possible that Nuri al-Maliki might lose his throne. As far as America is to be concerned, the Iraqi Government which we helped put in place does not want us to bring back ground troops, though they have requested aid. Now we wait and see what happens next.
Some info on the latest in Iraq:
Obama: Iraq needs help as ISIS threatens to seize more cities - CNN.com