The United States and Iran on Monday signaled increased willingness to work together to arrest the expanding Sunni insurgency in Iraq, with Secretary of State John Kerry openly suggesting such a collaboration would be constructive and another American official saying the subject could come up at talks this week on the Iranian nuclear dispute. Cooperation between the United States and Iran to contain the Iraqi crisis would represent the first time the two countries — estranged adversaries for more than three decades — have jointly undertaken a common security purpose since they shared military intelligence to counter the Taliban in Afghanistan in the aftermath of the Sept. 11 attacks 13 years ago. Mr. Kerry, in an interview with Yahoo News, called the advance by insurgents under the banner of the Islamic State in Iraq and Syria over the past week an “existential threat” to Iraq and suggested American airstrikes […]