The U.S. is at an important moment in the “serious business” of eradicating the threat of Islamic State militants in Iraq, Defense Secretary Ash Carter said Saturday in Baghdad.

Speaking with U.S. troops, Carter said the battle to retake Mosul is an important step in the campaign. “It's not a game,” he said. “It's necessary business because it's necessary to protect our country and to protect the rest of the world.”

“We will defeat ISIL,” he said, using the abbreviation for the Islamic State of Iraq and the Levant.

The ongoing fight in Iraq and Syria is just part of the larger task, Carter said. “We have to do it elsewhere in the world, and we have to protect our own country as well.”

Retaking Mosul is an important campaign because it involves a unified international coalition, with Iraq troops in the lead and with international partners including the U.S. in support. This unified structure has been in the making for more than a year, Carter said, predicting the force will continue moving through other Iraq cities.

“We know it's not going to be easy, but I'm encouraged by what I see so far,” Carter said. “It is proceeding according to plan, and we are on track.”

“This couldn't happen without the support of the international coalition,” Carter said. “That coalition couldn't exist without the United States, and that coalition couldn't be successful on the battlefield without the awesome prowess of the people in this room and in the U.S. military around the world.”

The level of U.S. involvement in Iraq over the long term remains unclear, Carter said. “We have discussed that with the Iraqi government,” he said. “In the end, it will be a decision that we make with the Iraqi government.”