It is the first time the military has entered the favelas or slums since Rio's governor requested federal help to stop the spread of drugs and violence this year and comes ahead of July's Pan American Games, when half a million visitors are expected to flood Rio de Janeiro.
The authorities are desperate to control the crime and violence so they are not scared away.
The Alemao, an area controlled by drug lords and gangs, is so dangerous that it is known as the Gaza strip of Rio de Janeiro.
In the last month alone 17 people were killed and 59 wounded there. Men, women and even children are being searched for drugs and weapons by the troops deployed on Wednesday.
Schools and shops were closed amid shootouts between soldiers and gang members.
The authorities are hoping the show of strength will not only assure visitors but also send a message to gangs operating in other Rio favelas that their time is up.
But some Rio residents are wondering if 450 soldiers are enough to combat crime that has riddled the area for decades.