On Thursday, Carter announced that all jobs in the U.S. military will now be open to women. Positions will no longer be closed exclusively because of gender; all members of the military will now be able to serve in any position for which they meet the standard. This decision will take effect on or around January 1, after the legally required 30-day congressional-review period ends.

Carter’s announcement is long overdue. As women in uniform have been fighting and dying in the wars of the last 15 years, their opportunities within the military have remained deeply constrained. Yet their remarkable performance in those same bloody conflicts has reinforced the beliefs of all who saw this day as both inevitable and right. And it has convinced many who were once skeptics.

This change will open the 10 percent of military specialties, totaling more than 200,000 individual positions, that had previously been available only to men—including many in infantry, armor, and special operations. Carter explained that opening these positions will ensure that the U.S. military will have access to the broadest possible range of talent, an edge that he considers critical to win the nation’s future wars and that is consistent with his broader Force of the Future initiative.

Carter clearly and unambiguously noted that there would be “no exceptions” to this policy. In January 2013, then-Secretary of Defense Leon Panetta announced that all military positions would be open to women, but gave the military services three years to assess the positions that were still closed, conduct studies and experiments, and request any permanent exceptions to that policy. None of the civilian-service secretaries requested any exceptions, nor did the uniformed chiefs of the Army, Navy, and Air Force, or the four-star general who heads the U.S. Special Operations Command. The commandant of the Marine Corps, however, requested two broad exceptions: one for specific operational specialties (including infantry officers, machine gunners, and special operations officers) and a second for whole types of units (including infantry regiments and reconnaissance battalions). The commandant at the time was General Joseph Dunford, and this request was one of his final actions before he assumed his current position as the chairman of the Joint Chiefs of Staff.

The Marines based this request on a two-and-a-half year, $36 million study they commissioned to assess an experimental integrated Marine unit. However, the methodology and conclusions of the study’s 978-page final report (which has not been officially released but is available here) sparked criticism, both for failing to establish clear job-specific standards and for focusing on the average performance of all women in the unit rather than the far more relevant metric of how well individual women performed. Secretary of the Navy Ray Mabus, who oversees both the Navy and the Marine Corps, publicly questioned whether Marine leaders had compromised the effort from the start. Mabus told NPR that the study “started out with a fairly large component of the men thinking this is not a good idea and women will never be able to do this. When you start out with that mindset you’re almost presupposing the outcome.” After studying the Marines’ request, Carter rightly chose not to grant those exceptions. The study, he explained, was “not definitive, not determinative,” because averages “do not determine whether an individual is qualified to participate in a given unit.”

The Marines’ basic argument was even more deeply undercut by the strong support for integration by the other services. In effect, the Marines were claiming that Marine riflemen, reconnaissance specialists, and machine gunners were somehow fundamentally different from their counterparts in Army infantry and paratroop units, Ranger battalions, Navy SEAL teams, and other elite special-operations units. The graduation of three women from the Army’s grueling Ranger school earlier this year further undermined the Marine Corps’ argument that women would not perform well under the toughest field conditions. Carter clearly found the Marines’ separatist logic unpersuasive. In his announcement, he deliberately reinforced that the U.S. military is a joint force that will operate with a single set of standards.

Carter also noted that approving the policy change is only a first step. His announcement made clear that he is focused on effective implementation. In his speech, Carter outlined seven principles to guide the implementation of the policy change across the force, and strongly emphasized that force effectiveness would not be compromised as the changes went into place on January 1. These include recognizing that changing ingrained cultures may prove challenging and that some segments of the services expect that standards and combat effectiveness will be compromised. Carter also tasked Deputy Secretary of Defense Bob Work and Vice Chairman of the Joint Chiefs of Staff General Paul Selva with leading the effort to monitor the implementation process and report back to him with any issues that arise, thus reinforcing the importance of getting the follow-up right.

But now that the decision has been made, effective implementation becomes primarily a matter of good leadership across the force. The Department of Defense stressed the importance of leadership in implementing the 2010 repeal of the “don’t ask, don’t tell” policy, which was widely seen as a smooth process that successfully integrated openly gay and lesbian service members into the force. That remains equally true for integrating women into these newly opened positions—and every service has plenty of leaders who can follow through to get that mission accomplished.

Despite the concern about the drawbacks of integrating units, though, this decision opens the doors for the U.S. military to draw upon the broadest possible assortment of talented young men and women, and put them in a meritocracy where their aspirations are only limited by their abilities, not their gender. This century’s conflicts will be won far more by out-thinking enemies than by out-muscling them, and including women in every part of the U.S. military makes America’s defenses stronger. This long-awaited and hard-earned policy change may turn out to be a vital advantage in America’s future wars.