Roger Cohen in The New York Times:
MILAN — The Paris slaughter claimed by the Islamic State constitutes, as President François Hollande of France declared, an “act of war.” As such, it demands of all NATO states a collective response under Article 5 of the North Atlantic Treaty. This says that, “An armed attack against one or more of them in Europe or North America shall be considered an attack against them all.” Alliance leaders are already debating what that response should be. Hollande has spoken to President Obama. Other NATO countries, including Germany and Canada, have expressed solidarity. Indignation and outrage, while justified, are not enough. The only adequate measure, after the killing of at least 129 people in Paris, is military, and the only objective commensurate with the ongoing threat is the crushing of ISIS and the elimination of its stronghold in Syria and Iraq. The barbaric terrorists exulting on social media at the blood they have spilled cannot be allowed any longer to control territory on which they are able to organize, finance, direct and plan their savagery.
…To defeat ISIS in Syria and Iraq will require NATO forces on the ground. After the protracted and inconclusive Western interventions in Iraq and Afghanistan, it is reasonable to ask if this would not be folly. It is also reasonable to demand – and many will – whether military action will only have the effect of winning more recruits for ISIS as more lives and treasure are squandered. Terrorism, the old nostrum has it, can never be completely defeated. Such arguments are seductive but must be resisted. An air war against ISIS will not get the job done; the Paris attacks occurred well into an unpersuasive bombing campaign. Major powers, including Russia and China, have vigorously condemned the Paris attacks. They should not stand in the way of a United Nations resolution authorizing military action to defeat and eliminate ISIS in Syria and Iraq. Regional powers, especially Saudi Arabia, have an interest in defeating the monster they helped create whose imagined Caliphate would destroy them.