Imperial Japanese Army VS US Gangs
Take, let's say a million IJA troops as of the start of WWII and have them set foot in America; with the mutual agreement of the US government they are tasked to wipe out all of the cartel and various gangs in all the major cities as of 2019
Gangs are independent actors, but may put their differences aside and band together to fight the IJA if reasonable. IJA has a year to completely annihilate them. Assume nobody can leave the USA, and every member can be tracked and found.
IJA starts from the West coast and works their way to the East. It is a surprise attack initially.
I'm by no means a gangs expert, but IJA would be tasked with wiping out:
MS-13
Aryan Brotherhood
Bloods
Crips
Hells Angels
KKK
Latin Kings
Mexican Mafia
Vagos
etc