Hi there.. I've been searching for a game for ages, and maybe I found my match, but on the whole internets, I only found one screenshot, here:
http://www.thelegacy.de/Museum/5167/
The description on Mobygames seem to fit it. It was part an isometric strategy game, where you could place your soldiers around a base on an island, and give orders to attack and so on, but then you had to change into different vehicles, and wreck the whole place. In the "vehicle" view you played the game from teh 1st Person View. It had Voxel graphics mixed with texture mapped polygons, if I remember correctly. It's been a long time, and the CD I had it on has wandered off to Nirvana quite some time ago. It was the first CD I ever burned
Maybe someone else can find some more screenshots, to confirm if it is indeed the game I am looking for.