From the Document: "The Constitution provides Congress with broad powers over the Armed Forces, including the power 'To raise and support Armies' and 'To provide and maintain a Navy.' In the exercise of this authority, Congress has historically shown great interest in military recruiting, which is critical to maintaining a fully manned and capable military workforce. Congress exercises a powerful influence on recruiting through its establishment of personnel end-strength levels for the Active Components and Reserve Components. Higher end-strengths generally require a greater number of new recruits, higher rates of retention among current servicemembers, or some combination of the two. Through its oversight powers, Congress monitors the performance of the executive branch in managing the size and quality of the military workforce. Congress influences the achievement of recruiting goals by the armed services in a number of ways:  authorizing military compensation packages that are competitive with civilian employers (e.g., basic pay, recruiting bonuses, educational and health care benefits);  establishing criteria that affect eligibility for enlistment such as age, cognitive, behavioral, and citizenship requirements; and  funding recruiting programs that provide for dedicated recruiters, market research, advertising, and military entrance processing stations. The policy levers most commonly used to manage recruiting include varying the number of recruiters, funding for advertising, and funding for enlistment incentives. When recruiting shortfalls occur, or are anticipated, Congress may elect to apply additional resources to these mechanisms. Likewise, when recruiting is expected to be strong, Congress may elect to shift resources away from these areas."
CRS In Focus, IF11147
Congressional Research Service: https://crsreports.congress.gov/