The US would still be stronger than either Russia or Brazil. It has better climate, more arable land, plenty of mineral resources, and higher population. A US without imperialism would probably be stronger than even the USSR at it's height since the americas are protected on both sides by ocean alleviating the need to maintain a massive military and the technological base is much higher.
Xi Jinping says West has long-term economic, military superiority
Kevin Green
Thomas Hernandez
Angel Rogers
Maybe the the center but it would still be big.