Since April 2018, the Department of Homeland Security (DHS) has submitted 33 requests for assistance (RFA) to the Department of Defense (DOD) for support to U.S. Customs and Border Protection’s (CBP) mission at the southwest border. DOD established six criteria for evaluating RFAs, which it documents in decision packages. When reviewing four selected decision packages, GAO found that DOD fully evaluated four of these six criteria. GAO found that DOD developed rough cost estimates that were not reliable. In addition, DOD did not fully evaluate the effect on military readiness of providing support at the time the Secretary of Defense considered DHS’s requests. Without reliable cost estimates and a timely readiness analysis, DOD is limited in its ability to evaluate the effect of supporting DHS on its budget and readiness rebuilding efforts.