The Obama-era policy of integrating women into ground combat units is a misguided social experiment that threatens military readiness and wastes resources in the service of a political agenda. The next defense secretary should end it.

In September 2015 the Marine Corps released a study comparing the performance of gender-integrated and male-only infantry units in simulated combat. The all-male teams greatly outperformed the integrated teams, whether on shooting, surmounting obstacles or evacuating casualties. Female Marines...