Cracker! - Cynthia Kadohata

Cracker works for the US army. She's a German shepherd dog trained to sniff out bombs, traps, and the enemy. The fate of entire platoons rests on her keen sense of smell.
Rick Hanski is headed to Vietnam, where he plans to prove his dad wrong - and everyone else who thinks he isn't tough enough for war. But soon he can't help wondering if they were right…
When Cracker is paired with Rick, she isn't too sure about her new handler. Is he up to the job? They need to be friends before they can become a team, and they've got to be a team if they want to get home alive…