WASHINGTON — Secretary of State John Kerry has agreed to testify before the House Committee on Oversight and Government Reform over the 2012 attacks on the American diplomatic mission in Benghazi, Libya. In a letter to Representative Darrell Issa, Republican of California, the committee’s combative chairman, the State Department’s assistant secretary for legislative affairs, Julia Frifield, expressed surprise at Mr. Issa’s May 14 subpoena “commanding the secretary of state to appear.” While she declined to make Mr. Kerry available next Thursday, the day the chairman had demanded, she offered two dates next month. “He will be fully occupied with critical...