Complex sentence: Policy differences over Frondizi-era oil contracts, which Illia rescinded, led the MID to actively oppose him, however, and he initially welcomed the 1966 coup against Illia.
Split into simpler sentences: Policy differences over Frondizi-era oil contracts, which Illia rescinded, led the MID to actively oppose him. Frondizi initially welcomed the 1966 coup against Illia.