The top U.S. commander in Afghanistan did not ask permission from the president before he ordered a massive conventional bomb to be dropped on a complex of tunnels used by ISIS fighters in a remote mountain region along the border with Pakistan, a senior defense official said Friday.

The GBU-43/B MOAB or massive ordnance air blast bomb, informally dubbed, the "Mother of All Bombs," is the largest non-nuclear bomb ever used in combat by the United States.

The official, who spoke on conditional of anonymity to discuss military plans, said Gen. John "Mick" Nicholson had brought the weapon into Afghanistan well before the inauguration of President Trump with an eye toward using it against this very target, a system of tunnels that ISIS-Khorasan fighters were using to move back and forth across the border undetected.

"He didn't ask permission. He didn't have to," said the official.

While President Trump said he has given the U.S. military "total authorization," the official said Nicholson ordered the attack under longstanding authorities that give U.S. commanders wide latitude to order offensive strikes against al Qaeda and affiliated terrorists.

In January of 2016, President Obama approved new rules of engagement to allow U.S. commanders to launch airstrikes against ISIS in Afghanistan, because of its links to al-Qaeda.

Then in June, Obama also lifted restrictions on offensive strikes against the Taliban, which until then, the U.S. only attacked to protect Afghan forces.

Nicholson was acting under his authority as the commander of the U.S. counterterrorism mission, known as Freedom's Sentinel, which is separate from the war between the Taliban and the Afghan government, which is under the umbrella of Operation Resolute Support.

"He had been looking at the target for months, and decided it was time to mix things up," the official said, while admitting the U.S. military did not fully anticipate how the first use of the massive bomb would capture the imagination of the public, and be interpreted as a signal to other nations from Trump.

"From his perspective, it was just another bomb," the official said. "We should have realized how it would be viewed. That's on us."