In both the nine hells and the abyss, souls are a source of power, its what gives them their spells and what not. There would definatly be an interest, and if one side got involved, the other side would too.
The way I would do it would be to open up a massive Gate or other permanent portal to said realm in some out of the way place in the world. Deep desert, the artic, bottom of the ocean, etc. I would establish a defensible base of operation, bring in adaquate forces, and then just wait. The chaos that would leak in natuarlly thru the portal would over time begin to alter the natural enviroment. No one would be the wiser until reports from far off lands start reporting psuedo-natural polar bears and fiendish camels.