From the time he lost the election on November 3, he repeated the claim that the election had been rigged and the Presidency had been stolen. This became a theme in his social media. He also called on his followers to "fight," to "stop the steal," on many occasions, over the months. For example:
"Wow! Thousands of people forming in Washington (D.C.) for Stop the Steal! #MAGA." "WE HAVE JUST BEGUN TO FIGHT!!!" December 12
We won the Presidential Election, by a lot. FIGHT FOR IT. Don’t let them take it away!" December 18.
"Big protest in D.C. on January 6th. Be there, will be wild!" December 19
"If the liberal Democrats take the Senate and the White House — and they’re not taking this White House — we’re going to fight like hell, I’ll tell you right now." January 4
He had a long history of encouraging violence at MAGA rallies dating back to his campaign of 2016. He told cops to rough up protesters. He said he wanted to beat one of them and told his followers to attack another. He offered to pay the legal expenses of anyone who attacked protesters at his rallies.
To demonstrate the extent of his approval of violence against his "enemies," he pardoned a former Navy Seal who had been charged by members of his own squad of murdering an unarmed and restrained prisoner. This violated the Uniform Code of Military Justice and the Code of Conduct of Seals. Many fellow seals testified against him and he was convicted by a Court Martial.
He made many dozens of other incendiary comments to his followers, leading up to the January 6, 2021, Treasonous Insurgency. That is why most of the people who were arrested for that act, who have commented, said they were told by him to do what they did. I take them at their word.