Hour of the Horde by Gordon R. Dickson

Annihilating everything before it, a horde of monstrous space travellers were advancing through the stars. And Earth lay in their route. To defend their home planets, the worlds that lay in the path of the monsters created a super defence force, asking each planet to contribute one especially talented warrior to help turn the invaders away. Miles Vander was Earth’s man, but when he arrived at the rendezvous point he found that he was included in the special task force of the less civilized defenders.
But in the contest of advanced nuclear weaponry and computer strategy, it turned out to be Vander’s group that had the special independent qualities and the raw courage to meet the challenge most effectively.