WASHINGTON - Defense Secretary Ash Carter said Thursday that he is opening all jobs in combat units to women, a landmark decision that ends a three-year period of research with a number of firsts for female service members and bitter debate at times about how women should be integrated.

The decision opens the military's most elite units to women who can meet the rigorous requirements for the positions for the first time, including the Navy SEALs, Army Special Forces, and other Special Operations Units. It also opens the Marine Corps infantry, a battle-hardened force that many service officials had openly advocated keeping closed to female service members.

The move signaled a formal recognition that thousands of women served, and many were wounded or killed, in the last 14 years of war in Iraq and Afghanistan.

"We are a joint force, and I have decided to make a decision which applies to the entire force," Carter told a news conference.

But he acknowledged some concerns. "Implementation won't happen overnight. And while at the end of the day this will make us a better and stronger force, there still will be problems to fix and challenges to overcome. We shouldn't diminish that."

Carter said that the chiefs of the Army, Navy, and Air Force all recommended that all jobs be opened to women. The Marine Corps recommended that certain jobs such as machine gunner be kept closed, but the secretary said that the military is a joint force, and his decision will apply to all services.

"There will be no exceptions," Carter said.

The decision is nearly three years in the making. In January 2013, the defense secretary at the time, Leon Panetta, announced that he was rescinding a longtime ban on women serving directly in ground combat units, but gave the services until this fall to research the issue.

Each service chief provided recommendations in September, and Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, reviewed them afterward. Carter told reporters at the Pentagon in September that he would be "very facts-based and analysis-based" once the decision reached him.

"I want to see the grounds upon which any actions that we take at the first of the year are going to be made," Carter said at the time. "That's the frame under which I'll be looking at."

The issue has at times opened an uncommonly public rift between senior military leaders. In particular, Navy Secretary Ray Mabus took issue with a Marine Corps study that found that the average woman struggled to keep up with men, according to a number of metrics. The study did not track individual performance, drawing fire from Mabus and others who favor full integration.

Dunford, who served until late September as commandant of the Marine Corps, recommended that the service keep its infantry and at least some reconnaissance units closed to women, Marine officials said. Mabus and Air Force Secretary Deborah James have generally shown support for opening all jobs to women who can meet the same standards as men, while senior Army officials have not disclosed their recommendations.

Over the last few years, women have steadily moved into many jobs previously open only to men, including on Navy submarines, in Army artillery units, and as Night Stalkers, the elite special operations helicopter crews, best known for flying the Navy SEALs into Osama bin Laden's compound in 2011.

Three women became the first to take and pass the Army's difficult Ranger course.

Rep. Tammy Duckworth (D., Ill.), one of the first Army women to fly combat missions in the 2003-11 Iraq war, welcomed the decision.

"I didn't lose my legs in a bar fight - of course women can serve in combat," said Duckworth, whose helicopter was hit by a rocket-propelled grenade. "This decision is long overdue."

Sen. John McCain (R., Ariz.), head of the Senate Armed Services Committee, said Congress will review the data and the decision.

The services will have to begin putting plans in place by April 1.

This article contains information from the Associated Press.