A battle of the brass has broken out in the wake of the Republican and Democratic party conventions, as one of the top generals supporting Donald Trump lashes out at retired Marine Gen. John Allen who delivered a tough-as-nails endorsement of Hillary Clinton in Philadelphia Thursday night.

“I honestly don’t know how John Allen can look at himself in the mirror and say why he supports Hillary Clinton,” retired Army Lt. Gen. Michael Flynn told "The Kelly File".

Both Flynn and Allen served in the Obama administration. But Flynn, who used to lead the Defense Intelligence Agency, has since become an outspoken critic of the president’s anti-ISIS approach – while Allen helped shape that strategy as the president’s special envoy for the anti-ISIS coalition.

Flynn, who spoke at the Cleveland Republican convention last week on behalf of Trump, zeroed in on that portion of Allen’s resume in challenging his credibility.

“General Allen as a retired officer was in charge of our current strategy for well over a year … and during that period of time the rise of radical Islamism and ISIS, you know, it exponentially grew,” he said. “The strategy that John Allen was in charge of … it’s a failed strategy.”

He said he was “a bit stunned” by his endorsement of Clinton, noting he and Allen both worked with the former secretary of state.

“I cannot see how John Allen can support somebody who perpetually cannot tell the truth,” he said.

The stinging criticism between senior retired military officers is unusual, even in a presidential campaign. But the tensions could build as each presidential candidate suggests the other would put national security at risk.

In Philadelphia, Allen, who previously led forces in Afghanistan, vouched for Clinton as the leader the country needs.

“We trust in her judgment. We believe in her vision for a united America, we believe in her vision of an America as a just and strong leader against the forces of hatred, the forces of chaos, and darkness,” he said. “I tell you without hesitation or reservation that Hillary Clinton will be exactly, exactly the kind of commander-in-chief America needs.”

Allen walked on the Philadelphia stage as Democrats tried to put a sharper focus on security issues, after being accused of glossing over ISIS and other terror threats the first two nights of their convention. Trump commented on the close of the convention by saying Democrats were living in a “fantasy world.”