I am late to the commentary on this topic, nevertheless the development is serious enough with consequences for many years to come.
The first question Americans want to know is why bother Islamic State when President himself has admitted in his speech that we are neither aware of any plans getting cooked to attack homeland nor we see any imminent capabilities developed by IS to attack America. Remember President Bush also argued that to prevent Mushroom cloud triggered by Saddam Hussain, he wanted America to take the preventive action. Brief answer to this question is though IS does have not any capacities to harm USA directly today; IS is getting stronger if un-checked and its intentions of harming everyone who are not Islamic in its own interpretation is clear enough to take precaution. Killing of two Americans and another of Scottish aid worker are proofs of these evil intentions. The other simple aspect is, sure America can wait till IS gets stronger and actually plots against America; and sure enough American military might can take off IS then too. But that would mean expending much more than when America can degrade IS much before at lower costs. Equally true is also the case that determining intentions of IS is not that complicated as determining whether Saddam Hussein possessed nukes. As President Obama very rightly said IS kills children, rapes women and in general prides itself in bringing Barbarism to Internet world. One of the most powerful statements in President's speech has been 'IS is neither Islamic nor it is a state'.
The important question is, given the danger IS poses; degrading it to a point where it does not pose any risk for USA and its allies is enough or eventual complete eradication of IS is needed. Because if later is the goal we are basically facing 80-20 rule: you expend 20% efforts to eradicate 80% of a terrorist organization like IS while you need 80% of efforts to eradicate remaining 20% of the organization. In other words, stopping IS is relatively less resource consuming but eradicating it completely will need lot longer time and more resources. Given that, it seems rather than claiming to say 'degrade and destroy IS'; it could have been lot more prudent for President to say our objective will be to ensure that IS or any derivative of that would not have any capacity whatsoever to harm Americans and legitimate interests of America. This is important because once you say you want to eventually 'destroy' IS, why not 'boots on ground' at sometime when it is needed? It is the quagmire Sec. Kerry finds himself in - whether it is 'war against IS' or not. But in common folklore, war means deploying all our resources until the adversary is completely and thoroughly vanquished. Folks on Right will find it a ludicrous idea to state that one can ever only 'degrade' organization like IS but not 'destroy completely'. In minds of American hawks, complete destruction of IS is the only path. However, prudence is not to set the debate of 'rallying America against IS' in those polemical terms.
When one frames the debate as 'complete destruction at all costs as the only choice'; America essentially commits herself again to a unity Iraqi State which will take over once Americans have done their job of eradicating IS. But we know from History that, once Zarqawi was vanquished and Sunni Awakening was complete; Shiite of Iraq simply squandered all that hard work. Regardless of all the talk of unitary Iraqi Government, for America to base its strategy to fight IS on the precondition of united Iraqi Government is essentially asking for more trouble. Safer for America is to pursue a strategy which does not have the pre-requisite of Iraq without sectarian fights. Iraq has shown the inability to grow leadership needed to remain together. But that does not mean, America let loose the pressure on Iraqi Politicians to overcome sectarian divide. That is good in itself and longer term. But what it means, there is no need to set goals which rest upon nation building; the exercise which has proven to be outside the releam of doable things for mighty America.
Powerful American role in international relations is badly needed. Degrading IS and cultivating conditions to eradicate it eventually (like President will rally the world opinion in containing human traffic of Jihadis flocking to Middle East war theater); is a legitimate exercise in America's power projection. So President Obama was right to extol America's exceptional duties and responsibilities there in his speech. One can perfectly imagine a world in absence of Uncle Sam, the menace of IS will be allowed to grow and then the world comes to deal with it haphazardly. Dealing with IS resolutely and effectively is something America can do and it needs to do it to protect her own citizens and her allies.
Many in this regard then question President Obama's decision 'now' to help Syrian Opposition apart from IS to wage a fight against Bashar Assad. These critics ask, if these resistance forces are good to 'arm now and support now'; why were they not good earlier when immediate response and intervention would have been lot more efficient? These critics have a point, but a simple answer to that is 'you take a risk appropriate to the context'. Back then IS did not warrant the risk of arming desperate Syrian opposition while today that risk pales on the background of 'critical mass' attained by IS evil. Hence, America would need to undertake all options available to stop IS, regardless whether that helps Bashar Assad or not.