The mentions of "a war veteran who was resurrected" and "a crab alien brain" made me think of Keith Laumer's A Plague of Demons (1965)—although there are also some differences from the terse description in the question.
Surprisingly, the novel does not have a Wikipedia page, although it was extremely influential. A review by Joe De Bolt and John R. Pfeiffer in Anatomy of Wonder, 2nd ed., 1981 (Neil Barron, ed.) describes the plot thus:
Aliens nurture wars on Earth to cultivate and reap a crop of 'computers' — human brains. John Bravis [sic.] is surgically converted to 'superman' to investigate. Caught, he finds his brain placed in control of the multi-ton tank fighting an unknown war on an unknown world. He rebels, frees fellow enslaved brains, and girds for endless war against the alien enemy.
The main character, John Bravais, is a secret operative on Earth, initially investigating some strange happenings surrounding a military conflict occurring in North Africa. It turns out that aliens are fomenting the conflict and harvesting the brains of the best human tacticians, which they transfer to their giant "BOLO" fighting machines, which are engaged in a much more serious war on another planet. Eventually, Bravais has is own brain captured, but he manages to use what learned before he was captured to turn the tables on his captors.
The crab alien brain is one of the most powerful of the enemies that Bravais has to face as the book comes to its climax:
It hung in the gloom of the dome, a bulging, grayish body in a cluster of tentacular members like giant angleworms, clinging to a bright filament depending from the peak of the onion-shaped dome. As I watched, it dropped down another foot, its glistening reticulated arms moving with a hideous, fluid grace. A cluster of stemmed sense organs poked from the upper side of the body—crab-eyes on a torso like a bag of oil. I recognized the shape of the creature; it was the one on which my borrowed mechanical form was modeled.