In some instances, if police demonstrate an overwhelmingly powerful presence either before a riot breaks out or at its very early stages, it is indeed possible for them to control the situation. But it is next to impossible to predict exactly where and when a riot is going to break out—by the time enough police are mobilized and in position, it’s usually too late.
Much more frequently, a show of force by police can make a situation far worse. The protests of the police killing of Floyd have unfolded in a manner that has become too common in recent decades. Protests emerge in reaction to a police killing. A march or rally proceeds peacefully until the protesters block a street, go down an unplanned route, or stay in an area longer than expected. The police intervene, the protesters resist, and then a confrontation ensues, typically spinning into something neither side expected. Police can quickly find themselves outnumbered, their show of force producing the circumstance it was designed to avoid.
Increasing the number of police who are trying to contain protests—particularly with inexperienced officers from outside the community who are not trained for crowd confrontations and lack the discipline necessary for such situations—often fans the flames instead of tamping them down. When the National Guard or other troops are called in to help police crowds, they usually escalate the combative nature of the situation, or play a central role in producing a violent confrontation in the first place.