He served with distinction in the First Samnite War under Aulus Cornelius Cossus Arvina. In 343 BC, Cossus, leading his army through the mountain fastnesses of Samnium, became trapped in a valley by the Samnites. Decius, taking 1,600 men, seized a strong point through which the Samnites were obliged to pass, and held it against them until nightfall; breaking through their lines, he re-joined the main body of the army, which had gained the summit of the mountain and relative safety. The army then swept into the Samnites, gaining a complete victory and the spoils of the enemy camp. For the rescue of the trapped army he was awarded the Grass Crown by both his own army and by the army he relieved.