Mitt Romney has struggled to explain why he omitted Afghanistan and the American troops stationed there during his big speech at the Republican National Convention. Ahead of Romney’s Tuesday address to the National Guard Association Convention, retired Gen. Wesley Clark said the exclusion shows Romney’s not ready to lead America’s military.

“Mitt Romney’s failure to mention Afghanistan and his comments about how his speech included the things you think are important is more than an omission,” Clark said on an Obama campaign conference call Monday. “It reveals a severe lack of understanding about the job as president, doesn’t reflect well on what kind of leadership you would bring and frankly it’s just unbecoming of someone who wants to become commander-in-chief.”

Romney has offered multiple explanations for failing to mention Afghanistan in his convention speech. First, he said the speech wasn’t “a laundry list” and that “you talk about the things you think are important” in speeches. A couple days later, in an interview that aired on “Meet the Press,” Romney said that a foreign policy speech he gave several days before his Tampa address — in which he devoted 15 seconds or so out of 16 minutes to Afghanistan — was enough.