Defense Primer: President’s Constitutional Authority with Regard to the Armed Forces

"Article II, Section 2, Clause 1
The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States; he may require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any Subject relating to the Duties of their respective Office, and he shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.

Commander in Chief:
The Constitution makes the President Commander in Chief of the Armed Forces, but does not define exactly what powers he may exercise in that role. Nor does it explain the extent to which Congress, using its own constitutional powers, may influence how the President commands the Armed Forces. Separation-of-powers debates seem to arise with some frequency regarding the exercise of military powers..."
Presidential authority and armed forces

Legislation Limiting the President’s Power to Use Nuclear Weapons: Separation of Powers Implications

"Recent proposed legislation that would place limitations on the President’s power to employ nuclear weapons has prompted interest in questions related to the constitutional allocation of power over the United States’ nuclear arsenal. This memorandum examines the constitutional separation of powers principles implicated by legislative proposals that restrict the President’s authority to launch nuclear weapons.1
I. Recent Legislation and Proposals to Restrict First-Use of Nuclear Weapons
 Legislation proposed in the 115th Congress would limit the President’s ability to order a “first-use nuclear strike.”2 On January 24, 2017, identical versions of a bill titled the Restricting First-Use of Nuclear Weapons Act of 2017 (Restricting First-Use Bill) were introduced in both chambers of Congress. The Restricting First-Use Bill would prohibit the President from using the “Armed Forces of the United States to conduct a first-use nuclear strike unless such strike is conducted pursuant to a declaration of war by Congress that expressly authorizes such strike.”3 The term “first-use nuclear strike” is defined as an “attack using nuclear weapons against an enemy that is conducted without the President determining that the enemy has first launched a nuclear strike against the United States or an ally of the United States.”4 While some have advocated that the United States adopt a broader “no-first-use” policy and pledge never to use nuclear weapons first against a nuclear-armed adversary,5 the Restricting First-Use Bill would address the President’s ability to act as the sole decision maker when authorizing use of the nuclear arsenal. The Bill would not address overall U.S. policy on first-use, nor would it modify directly the technical mechanisms through which nuclear weapons are employed..."
President and nuclear weapons

Can Congress Limit the President's Power to Launch Nuclear Weapons?

"Recent legislation proposed in the 115th Congress intended to limit the President's ability to launch nuclear weapons has prompted heightened attention on Congress's constitutional power to control the nuclear arsenal. As outlined in earlier CRS products, the Constitution allocates the authorities necessary to conduct war and other military operations between Congress and the President. But the precise contours of each branch's respective powers have been the subject of debate since the founding era. Moreover, courts traditionally have been reluctant to resolve wartime separation of powers disputes between the legislative and executive branches, often dismissing these cases on jurisdictional grounds without reaching the merits of the constitutional challenges.

Against this backdrop of uncertainty, commentators have reached dramatically differing conclusions on the constitutionality of proposals to restrict the President's power over the nuclear arsenal. Proponents of congressional authority reason that Congress's many enumerated war powers—including the power to power "raise and support Armies" and "provide and maintain a Navy"—necessarily subsume a lesser authority to define how the President may utilize the forces and weapons that Congress has provided. But proponents of executive authority often argue that such restrictions would unconstitutionally infringe on the President's "commander in chief" power to make tactical decisions on how best to subdue an enemy..." 
President and nuclear weapons