on opposite sides of the barricades

The idiom has been in use for at least a century http://www.marxists.org/archive/lenin/works/1908/jul/23.htm https://web.archive.org/web/20060618034021/http://www.fmep.org/reports/special_reports/no10-winter2000/01-clinton_administration.html. It probably derives from a literal observation of political protests, where government buildings were insulated with barricades, and the government was said to not represent the people because they and the people were on opposite sides of the barricades.

• diametrically opposed