Answer :
On January 6, 2021 a large riot had gathered and stormed the capitol building to protest against the presidency. At the same time as the rioters were entering the capitol building, congress and lawmakers were inside counting the votes for this past election. The capitol were not prepared for the rioting, and had little law enforcement there to stop such a large group.
As the rioters walked up the steps of the building, officers were there trying to resist the group, but didn’t have the manpower necessary. This led to the people being able to overpower law enforcement and walk into the building, forcing their way into the capitol. Reinforcement was called, the national guard showing up a while after, and standing on guard to keep watch on the capitol building after the fact.