When Roman armies headed to Germany on their way to conquer Europe, they brought the Rottweiler's ancestors with them. 

Because there were no refrigerators at the time, the soldiers traveled with their cattle, rather than slaughter the cows for meat before the journey. 

And naturally, they needed assistance keeping their cattle in line. 

The Rottweiler’s ancestors were the perfect dogs for the job thanks to their endurance and strength.