Another trend that has sapped Congress’ influence is the decline of congressional expertise on foreign policy and national security. Simply put, legislators used to know more about foreign policy than they do now. Greater expertise strengthened Congress’ formal and visible role, since committees could engage in greater oversight of the executive branch. Expertise also reinforced Congress’ invisible means of constraining presidential power. Presidents had to think about how a seasoned committee chair or member would assess a policy. During his initial escalation of the Vietnam War, for example, President Lyndon Johnson was careful to maintain the support of powerful committee chairs, such as Senator J. William Fulbright, who led the Senate Foreign Relations Committee from 1959 to 1974. Fulbright shepherded the Gulf of Tonkin Resolution through the Senate in 1964, but two years later, his probative hearings helped shift public opinion against the war.
https://www.foreignaffairs.com/articles/2018-08-13/unconstrained-presidency?cid=soc-tw&pgtype=hpg