The desire to move across North America did not come exclusively from the executive branch. Voices in both branches wanted continental dominance, and they won out over those whose preference was for a small republic. Even in the nineteenth century, when the United States was considered isolationist, these acquisitions and the Mexican-American War showed a clear victory for those with grand ambitions for the United States and its place in the world.
During this time, we do not see presidential unilateralism. If a president wished to initiate a military operation, he would ask Congress for permission to purchase territory or start a war—in Jefferson’s case, he asked for forgiveness after a fait accompli. Congress, in turn, would engage in a meaningful debate about the merits of the action, and provide him with permission or deny it. Subsequently, if a war effort proved questionable or problematic, they would again debate the merits of the prosecution of the war and hold the executive accountable. During an operation, spending would increase, and a president would have more power. At the conclusion of the operation, the military would draw down, and Congress would return to its coequal status.
This changed with the Spanish-American War. President William McKinley assumed a great deal of power—at the expense of Congress—and military spending started ratcheting up. The healthy push and pull over the eighteenth and nineteenth centuries between the branches began eroding then continued to do so through WWI. The healthy balance never returned after WWII. The Cold War and the War on Terror then exacerbated an already problematic relationship. Congress did not have the incentives to reassert its coequal status. This is certainly problematic for the separation of powers and the health of the constitutional system; there is a bigger problem. Congress facilitated unilateral presidential decision-making when it comes to the military without much oversight from the people, the courts, or Congress. This lack of deliberation and accountability has led to the operationalization of bad policy. This policy creates new security threats rather than diminishing them, and we have seen decades without a coherent grand strategy. Despite mountains of evidence proving the need for a more assertive Congress, presidents continue to make the same kind of mistakes in military engagements, large and small.