Novelist and Claremont Institute senior fellow Mark Helprin says, yes, take Mosul, but ISIS isn't the real threat in the Middle East. The real threat is newly-empowered Iran. Iran, according to Helprin, uses ISIS as a matador's cape–it distract us.
Writing in today's Wall Street Journal, Helprin argues:
Much more befitting of the power and history of the U.S. and its allies would be to sever and destroy the toxic, threatening bridge that Iran has built from Afghanistan to the Mediterranean, with, astoundingly, the patronage of the president of the United States. Anchored by soon-to-be-nuclear Iran, an integrated politico-religious-military front including Shiite-directed Iraq, Syria and Lebanon will emerge in the near future if current trajectories remain undisturbed.
This entity will have a population almost half that of the United States; the immense oil wealth of Iran and Iraq; ports on the Mediterranean, the Persian Gulf, and Indian Ocean; nuclear weapons; ICBMs; and, until it will no longer need Russia, for which it has no brief, the mischievous and destructive cooperation of Vladimir Putin.
If, under the discipline of an Iran drunk with its successful bamboozling of the West, this power turns its eyes south to Jordan and Saudi Arabia, the Middle East will be entirely transformed. When Iran crosses the nuclear threshold, so will Saudi Arabia.
The Shiite population of the Gulf States will be emboldened. Egypt will have to choose appeasement or standing with its Sunni co-religionists. How these elements would sort themselves out cannot be known in advance, but keep in mind that the cultures and governments of the Islamic world are hardly shy about violence and war.
President Obama has boosted Iran, which escaped sanctions and got billions of dollars under the current administration, for reasons that still remain unclear. And, as Helprin points out, Iran will not honor the nuclear deal (or interpret it in novel ways), while the U.S. releases money for the terror-exporting regime.
Helprin doesn't say what should be done about Iran, but he makes it crystal clear that to a large extent the empowerment of Iran is something the U.S. did.
Iran has received scant attention in the current presidential race (though Mrs. Clinton supports the nuclear deal, while Donald Trump does not).
I urge you to read the entire piece (which also argues that ISIS could be crushed if the U.S. and our allies were determined to do so).