Fearing an attack across the canal would be costly, Lieutenant General Monash decided to assault over the top of the tunnel. With the Australian Corps exhausted from almost continuous operations since 8 August, Monash had only two divisions in a reasonable state for combat – the 3rd and 5th – and was thus reinforced with the 27th and 30th United States Divisions.
The plan was for the numerically superior Americans to breach the Hindenburg Line above the tunnel, and another defensive line a kilometre to the rear.
The Australians would then pass through and assault the Beaurevoir Line, another four kilometres back. The attack would be supported by 90 tanks and heavy artillery concentrations.
The inexperience of the Americans was telling. An operation launched to secure the start line on 27 September 1918 was unsuccessful due to their failure to properly clear dugouts and trenches.
The same mistakes were repeated by the 27th Division when the actual attack was launched two days later. The 3rd Australian Division, trying to advance to its own start line, became embroiled in the fight for the Americans’ first objective.
With all of the tanks destroyed or disabled, and the uncertain position of the forward troops preventing the use of artillery, the battle degenerated into a struggle for individual strong points, fought with bombs, bayonets and Lewis guns, that lasted for another three days.
The 30th American Division, attacking further to the south, was more successful, enabling the 5th Australian Division to pass through and capture the heavily fortified village of Bellicourt. Their progress, however, was hampered by the lack of it in the north.
Even further south the British 9th Corps had managed to cross the canal, breach the Hindenburg Line, and begin advancing upon the Beaurevoir Line.
This action threatened to outflank the positions along the tunnel, resulting in a gradual German withdrawal that began on the night of 30 September. By 2 October a gap of approximately 17 kilometres had been opened in the Hindenburg Line. The operation had cost the 3rd and 5th Australian Divisions 2,577 casualties. Photo: Australian and American Troops at the southern entrance of the St. Quentin Canal Tunnel, captured by 30th American Division on 29th September 1918. More; http://ow.ly/SMcbK