An Iraqi security forces member with his weapon takes position as people, who fled from the violence in Mosul, arrive in their vehicles at a camp for internally displaced people on the outskirts of Arbil in Iraq's Kurdistan region June 14, 2014. Reuters

That's because the American public is strongly opposed to spending more money, let alone troops, in a country that most consider a failure. According to a January Pew Research Center survey, 52 percent of Americans think the United States failed to achieve its objectives in Iraq. The quick fall of northern Iraq is a testament to this failure. It also has very real consequences for Americans. Read More Iran and the US: Can frenemies unite over Iraq?

Here are five ways the collapse of Iraq could hit here at home. The threat to America and other Western nations increases dramatically. There have already been documented cases of American citizens fighting with ISIS. If the group were allowed to control southern Syria and northern Iraq, it would control territory larger than many countries. It could even form a government to govern the entire area. This would effectively make this part of the world a safe haven for terrorists, just as Afghanistan was under the Taliban. These terrorists are already holding Western weapons abandoned by Iraqi troops. Also, because some of ISIS' members hold Western passports, they would be more able to travel to the United States and Europe. Sooner or later, they would strike here. It could derail the world economy. The crisis in Iraq has already caused an oil shock, with energy prices spiking as ISIS advanced. If the crisis continues, you'll eventually be paying more for gas. But there is impact beyond the pump.

Volunteers, who have joined the Iraqi Army to fight against predominantly Sunni militants from the radical Islamic State of Iraq and the Levant (ISIL), carry weapons during a parade in the streets in Al-Fdhiliya district, eastern Baghdad June 15, 2014. Reuters

"The worst case scenario is that we see production from Iraq slip down to levels in the last Gulf war, then oil could spike $20 a barrel very quickly," Ole Hansen, vice-president and head of commodity strategy at Saxo Bank told The Telegraph, a British newspaper. "In that scenario, the entire economic recovery, which is still fragile, could stall, and we could even slip back into recession in some regions." It makes the United State look terribly weak. The Obama administration effectively abandoned Iraq in 2011, trusting the Maliki government to protect the gains it made in a long, deadly war. Read MoreMilitants say theyexecuted 1,700 Iraqi soldiers The loss of nearly half of the country shows that this trust was misplaced. The United States spent more than a trillion dollars in Iraq, and has provided the Iraqi government with tens of billions of dollars in aid. If the country fails, it would arguably be one the worst foreign policy disasters in the history of the country. It could decimate military morale. The importance of the money lost in Iraq pales in comparison to the human cost American soldiers paid. According to the Pentagon, 4,424 died fighting in Iraq. Nearly 32,000 Americans were injured, some with debilitating wounds like the loss of limbs.