USA weakened Iraq's army and made it a dependent army, such a weak army could not take serious action against ISIS.
On the other hand, ISIS used the hater which was caused by USA to attract Muslims to fight against their enemies.
USA is steal weakening Syria's army to slay Assad. The army which directly fights with ISIS!
The United States foreign policy has been of hypocrisy of the recent decades. First we supplied arms to Hussein to fight Iran. Then we got butthurt and disposed him to find alleged arms we may have supplied to him. If Hussein was still here, ISIS would not be a thing. Yes he was brutal, but he wasn't the same as ISIS. After we removed him and installed a different government, a lot of Hussein's ex-generals and stuff, with their warfare knowledge, went to, well, ISIS. There are many other factors too though.
The Bush Administration's wanton greed and complete lack of a plan for a post-Saddam Iraq created a power vacuum that ISIS was able to easily fill. A very similar situation is currently occurring in Syria, with a weakened Assad regime succumbing to ISIS. One of the keys to defeating ISIS will be making sure we have an actual plan for a post-war Middle East, and making better decisions to ensure anti-American sentiment doesn't coalesce into a violent terrorist organization.
The U.S has long been diving into situations that they do not need to be involved in. Many times the U.S has been told by those living in Iraq that they are not needed and to get out and yet more and more troops are being sent there. The U.S sends weapons to Iraq and then only expects them to be used against the bad guys. Well in Iraq the U.S is part of the bad guys. If the U.S were to leave Iraq now though the situation would be intensified as it would seem as "ISIS" has won.
I know that the intefereing with sunni - shia conflict, and the stationing of troops at Saudi Arabia has been a frustrating moment for them. However the isis rise again because we withdrew troops just like Vietnam. Isis is the second Al-Qaeda. Like in vietnam the VCs became won when we stopped interfering. It's happening all over again in Iraq.
The U.S. Invasion of Iraq is generally cited as one of the reasons for the rise of ISIS, however it is not completely possible to agree with the view. International relations experts working on the subject and studying the causes would definitely point to several other historical and political aspects as well.
In my opinion while the U.S. invasion of Iraq was more than likely a contributing factor to the rise of ISIS, it can most certainly not be called the sole blame. ISIS is huge and would have risen whether the U.S. invaded Iraq or not. It is not right to pin the blame on one event.