Stephen Colbert welcomed Bill O'Reilly to The Late Show on Monday, where the pair had a fascinating debate about gun control, the war on terror and Donald Trump in the wake of the deadliest mass shooting in America.

O'Reilly declared his hope that the gay club attack in Orlando, which left 49 dead and 53 wounded, would inspire Congress to declare war on Islamic terrorist groups, while simultaneously admitting that such a declaration would do nothing to stop a lone gunmen in the United States, such as the Orlando shooter Omar Mateen.

"So why would we then declare war based on this?" Colbert probed.

O'Reilly reasoned that declaring war would expand NATO's power, and that it would then be able to "destroy" ISIS. "Once the world sees that these people are being annihilated — and I mean annihilated — [ISIS'] whole propaganda ... vanishes."

Bill O'Reilly and Stephen Colbert, The Late Show with Stephen ColbertBill O'Reilly and Stephen Colbert, The Late Show with Stephen Colbert

Colbert also asked what O'Reilly thought of Trump's tweet following the massacre, in which he wrote: "Appreciate the congrats on being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!"

O'Reilly matter-of-factly called it a "political response," and seemingly shrugged off Trump's attempt to use terrorism as a tool to bolster his popularity as Trump doing what's necessary during a presidential campaign.

"Trump's betting that the country now wants a real avenger. That's what he's betting," O'Reilly said. "Mrs. Clinton right now is very tied into the Obama legacy right now, because she needs him right now, to campaign. So it is political advantage to Trump on this one, if that's what we're talking about. He has the advantage on this one and he wants to be president, so he's going to use it."

(Full disclosure: is owned by CBS.)