Boxers are friendly, lively companions that are popular as family dogs. Their suspicion of strangers, alertness, agility, and strength make them formidable guard dogs. They sometimes appear at dog agility or dog obedience trials and flyball events. These strong and intelligent animals have also been used as service dogs, guide dogs for the blind, therapy dogs, police dogs in K9 units, and occasionally herding cattle or sheep. The versatility of Boxers was recognized early on by the military, which has used them as valuable messenger dogs, pack carriers, and attack and guard dogs in times of war.