The army was forced to abandon the project and the corpses who had already been brought back could not be killed ,so they were contained in secure barrels.

After Frank hits the tank, believing that it is stong enough to withstand the blow, a crack in the tank releases the 2-4-5 trioxin gas and reawakens the Tarman while melting its skin. Tarman came out of the barrel and caused enough damage to start a zombie apocalypse.