 Later in the year , Maximian led a surprise invasion of the Agri <unk> – a region between the upper Rhine and upper Danube deep within Alamanni territory – while Diocletian invaded Germany via Raetia . Both emperors burned crops and food supplies as they went , destroying the Germans ' means of sustenance . They added large swathes of territory to the Empire and allowed Maximian 's build @-@ up to proceed without further disturbance . In the aftermath of the war , towns along the Rhine were rebuilt , bridgeheads created on the eastern banks at such places as Mainz and Cologne , and a military frontier was established , comprising forts , roads , and fortified towns . A military highway through <unk> ( Tournai , Belgium ) , <unk> ( Bavay , France ) , <unk> <unk> ( Tongeren , Belgium ) , <unk> <unk> ( Maastricht , Netherlands ) , and Cologne connected points along the frontier . 
