The militants don't actually have many soldiers mind. I distinctly remember 5000 being said at some point, though I would take that with a grain of salt until I can get actual numbers. But air-strikes are pretty damned helpful, especially given the weapons the militants have captured; and remember that during few day period when it looked like Obama was going to order airstrikes, ISIS was throwing a fit in Syria trying to get all it's stuff underground.
Of course, I think the biggest conditional is the Iraqi government. There army isn't doing so hot (although it seems very likely the troops closer to Baghdad are more loyal. The stories of massive executions of former gov't employees probably helps.), and much of their new strength in recent days has come from Shiite militias (who appear for the most part to be working in tandem with the government). And likely one of the big reasons for the insurgency and the army's failure to stop it is the treatment of Sunnis (most areas captured by the insurgents so far have been Sunni dominated). If Iraq represses them now, it puts pressure on American officials to not help them.
Germany did provoke the US with the zimmerman telegram and the Lusitania sinking. If they didn't want the US to join into the war, they shouldn't have goaded us.
The Lusitania was a legitimate target. It was carrying large supplies of ammunition ( amongst which 90 tons butter and lard destined for the Royal Navy Weapons Testing Establishment in Essex) in deliberate breach of the Cruiser Guidelines.
Hell, there are theories, which I would not as easily dismiss, that the ship was deliberately placed in harms way in order to provoke a Germans submarine attack, as to draw the US into the war. Would explain why it was not diverted from it's course (the subs location was known), why it was not escorted, and some other things.
It wasn't marked as such though. It's like killing random people and finding out they were bad: It doesn't justify the earlier action. It was not a military ship, they were required to warn them before firing.
The Lusitania was not the reason the US intervened though. I mean it pissed Americans off but there was a big two-year gap between that and US intervention. It was the resumption of unrestricted Submarine warfare (and the following sinking of over 5 American merchant ships in rapid succession) and the Zimmerman telegraph at
just that moment that finally pushed the public (and Wilson) to intervene.