Standing Armies and Riots

"The Army is a broadsword, not a scalpel. Trust me, senator, you do not want the Army in an American city."
--General William Devereaux (The Siege)

President Trump has threatened to send in US military troops where states refuse to take measures to appropriately defend the lives and property of their residents from rioters. Constitutionally, the president has no leg to stand on.

Article 4, Section 4 states that the federal government "shall guarantee to every state in this Union a Republican form of Government"(i.e., the 'Guarantee Clause'). Further, the federal government shall protect each of [the states] against Invasion," but clearly this 'Invasion Clause' is aimed at penetration from external enemies.

The article also states that the federal government shall protect each state from "domestic Violence," but only upon the request of the state legislature or the governor if the legislature cannot be convened. This 'Protection Clause' is clearly aimed at addressing internal uprisings that are presumed to be each state's primary responsibility to handle.

Stated differently, states must request the assistance of US troops to quell internal violence. The president cannot unilaterally deploy them.

An obvious question to our founding ancestors is this. If life and property are among the inalienable rights endowed to each person, and it is the responsibility of a just government to protect those rights, then shouldn't the federal government proactively intervene when states refuse to take appropriate protective measures?

The founders would likely say 'no.' If the federal government is permitted to forcibly intervene in state affairs whenever it deems that a 'crisis' exists, then the authority to deploy unilaterally 'standing armies' ultimately leads to discretionary rule, and the usurpation of that other inalienable right granted to all: liberty.

How, then, are people to protect themselves against threats to life and property if state and local government refuse to adequately do so. The founders answered this with the Second Amendment. Individuals are justified to arm themselves to defend their lives and property against aggression by others when government agents can't, or won't, be there to do so.