 Mines, demolitions, and strongpoints usually protected by a tank or an assault gun were the main obstacles.By nightfall, both the 3rd and 45th Divisions were well across the German frontier, scarcely more than a stone's throw from the outposts of the Siegfried Line, and the 100th Division, relieved at Bitche by a follow-up infantry division, had begun to come abreast.Fighter-bombers of the XII Tactical Air Command again were out in force.