The Aldermen's Wars occurred when the Italians tried to take political control of the (then) 19th ward from John Powers and his connections. The Italians had recently started to become the majority in the 19th ward and Powers would eventually see the writing on the wall.

More detailed background can be found over at Allan May's 2009 article Unione Siciliana part1 - Crime Magazine

The problem is that article (nor any other) ever touches on who was the actual muscle protecting Powers?

Assuming it's the Genna's involved in this and backing D'Andrea, can it be anyone other than the North Side gang (O'Banion/Drucci/Moran/Weiss etc), given that they would eventually be the ones to kill the Gennas over them trying to encroach North Side territory? While we have exhaustive information on the Capone era there is precious little on the early Chicago wars in both Little Italy and Little Sicily and the Aldermen's wars in general.