As Canada dropped its first bombs over Iraq on Sunday, the Pentagon's press secretary says the United States is preparing "for a years-long effort" in the fight against Islamic State militants.

Pentagon Press Secretary Rear Adm. John Kirby made the comments to CTV’s Question Period earlier this week, shortly before Canadian CF-18s dropped 500-pound laser-guided missiles near the city of Fallujah.

While two CF-18s flew Canada's first combat mission over Iraq on Thursday, they didn’t drop any bombs. That changed Sunday, when Canada took part in an approximately four-hour mission alongside its coalition partners, led by the U.S.

When asked how long the anti-ISIS fight may take, Kirby responded that at the Pentagon, "we are preparing ourselves for a years-long effort."

Success against ISIS will not be measured in the number of militants that coalition forces kill, he said. Rather, the goal is to destroy the terror group's ability to function, as well as destroy the "attractiveness of this warped, barbaric ideology."

"This isn't just about killing individuals," Kirby said. "This is about degrading and destroying the capabilities of this group to exist as an entity and to have the influence that they continue to have."

Asked whether U.S. officials hope Canada will renew its six-month mission, which is limited to airstrikes against ISIS targets in Iraq, Kirby would only say that "we look forward to working with the Canadian military, as we always do."

Chain of command

Last month, six CF-18s, two CP-140 Aurora surveillance planes and one C-150 refuelling jet arrived at an undisclosed air base in Kuwait, from where they are launching airstrikes as part of the U.S.-led coalition.

In his interview with Question Period, Kirby offered some details about how Canadian pilots will receive their orders.

Targets are selected by personnel at the Combined Air Operations Centre, which is operating in the region under U.S. Central Command.

Each day, the Combined Air Operations Centre assigns what is called an "air tasking order," which outlines what missions the aircraft will fly, where and with which weapons. Targets are assigned according to the available aircraft that day, Kirby said.

Canada's Chief of Defence Staff, Gen. Tom Lawson, has said that Canadian commanders can veto any assignment given to Canadian pilots if they feel it is outside their mandate.

According to Kirby, countries are not assigned to cover specific regions. Rather, missions will depend on the targets.

"The way we've been conducting these strikes is not based on geographic zones necessarily as assignments to different countries, but rather going after targets of relevance and making sure we choose the proper asset from the air to go after a proper target," Kirby said.

"So it's really not driven by lines on the map."

While Canada's mission only calls for Canadian planes to hit targets in Iraq, Prime Minister Stephen Harper has left the door open for airstrikes in Syria only if permission is granted by the Syrian government.

Other coalition forces are hitting targets in Syria to undermine ISIS militants' ability to organize, recruit, arm and train fighters, Kirby said. In Iraq, forces are focused on "dynamic targeting."

"We're going after actual targets that are either threatening or about to threaten Iraqi and Kurdish security forces," he said.

Well over 400 airstrikes have been conducted in total in both countries, Kirby said, and they continue every day.

"We've been saying from the beginning that this is going to be a long struggle."