Why the US used the MOAB bomb to target ISIS fighters in Afghanistan

On Thursday, the U.S. military dropped a never-before-used 22,000-pound bomb nicknamed the "mother of all bombs" on ISIS fighters in Afghanistan.

Why was now the right time to deploy the massive weapon? Here's what you need to know:

Why was the bomb used?

The top U.S. commander of American forces in Afghanistan said the bomb was deemed the best to target the ISIS-K's tunnel complex in Nangarhar province, located in eastern Afghanistan.

Formally known as the GBU-43, or massive ordnance air blast (MOAB) bomb, it is the largest non-nuclear bomb ever used in combat by the U.S. military. Thirty-six ISIS fighters have been killed, according to the Afghan Ministry of Defense. As of now, there is no evidence of civilian casualties and no reports of civilian casualties, but an assessment of the bomb damage to the area is still ongoing.

According to U.S. officials, the number of ISIS fighters in eastern Afghanistan is estimated to be between 600 and 800 -- a decrease from the estimated 3,000 operating in that area in late 2015.

The MOAB "is primarily intended for soft to medium surface targets covering extended areas, targets contained in an environment such as caves or canyons, clearing extensive mine fields, and for psychological effects," according to Ann Stefanek, a spokesperson for the Air Force.

"This was the right weapon against this target," Gen. John Nicholson, commander of U.S. Forces - Afghanistan, said in a press conference in Kabul Friday.

"The enemy had created bunkers, tunnels, and extensive minefields and this weapon was used to reduce those obstacles so we could continue our offensive," he added.

Nicholson further explained that the purpose of the operation was to "eliminate" ISIS-K's "sanctuary."

Why was it dropped on Thursday?

Thursday was simply the proper tactical moment for this particular target, according to Nicholson.

"In regard to timing, it's when we encountered this target on the battlefield," Nicholson said.

"It is not related to any outside events other than our focus on destroying Daesh in 2017," he added, using another name for ISIS.

The U.S. military has been conducting operations against ISIS in Afghanistan for the past year, specifically in southern Nangarhar since March, and has reduced the threat to a smaller area by roughly "two-thirds," Nicholson said.

"However this was the first time that we encountered an extensive obstacle to our progress," Nicholson explained.

The obstacles, according to Nicholson, included IEDs (improvised explosive devices), and the presence of tunnels and caves.

The U.S. military's planning to use the MOAB bomb goes back several months to the Obama administration, and Department of Defense officials were aware of the intent and plans to deploy the weapon.

Does the use of the bomb send a message to North Korea?

With North Korea reportedly preparing to conduct a nuclear test this weekend, President Donald Trump was asked whether dropping the bomb sends a message to the eastern aggressor.

"I don't know if this sends a message. It doesn't make any difference if it does or not. North Korea is a problem. The problem will be taken care of," Trump said Thursday during a meeting at the White House.

Col. Steve Ganyard, a retired fighter pilot in the Marine Corps and current ABC News contributor, said on "Good Morning America" Friday suggested it certainly raises an alarm.

"It sends a message to ISIS but I think the real question here [is] ... 'Was that heard on the other side of the world in Pyongyang and will it make an impression on Kim Jong-un?'" Ganyard said. "I think so."

Will the U.S. use this bomb again in the future?

"That depends on the tactical situation," Nicholson said. "We employed this weapon for a specific tactical solution to a particular military problem and that determines when and how we operate when and how on the battlefield."

ABC News' Luis Martinez contributed to this report.