In February 1861, as Congress gathered to certify the election of Abraham Lincoln, southern sympathizers in both houses tried to hold up the proceedings every way they could. Rumors swirled that the Virginia militia were on their way to blow up to the Capitol. That didn't happen, but an angry mob did gather. They probably would have stormed the building if not for the decisive actions taken by General Winfield Scott, an old man who had been given his first commission by Thomas Jefferson. Fearing trouble, Scott brought two companies of troops to Washington weeks in advance by announcing that he was going to hold a military parade to welcome the new president.
Scott stationed his troops and two canons at the building and threatened to strap the first rioter who tried to enter to the muzzle of a 12-pounder and "manure the hills of Arlington with the fragments of his body." Nobody tried to force his way in.
When Senator Louis Wigfall of Texas asked Scott if he would dare arrest a senator for treason, Scott replied, "No! I would blow him to hell!"
On the one hand, it worked, the certification proceeded without violence, and Lincoln assumed office. On the other, Scott's mailed fist approach was part of the process by which the nation descended into war.
We have seen recently that when the police deploy with the numbers and equipment they think are necessary to guarantee peace, the show of force can be seen by protesters as an act of violence in itself. So I get why various powers in Washington were reluctant to deploy massive force on January 6. In retrospect, they should have, but let's not pretend that would have been cost free.