I'm not a military strategist , so I may be completely mistaken here.
It seems to me that "it" in the last sentence refers to the "offensive last year which was impossible", not to the road.
Here is the situation as the author describes it:
Last year the author's side could not advance on the enemy's (Austrians') side because
1) There was a river separating the two armies.
2) On the author's side of the river there was only one narrow road leading to the river bank.
3) The road was under enemy fire for nearly a mile
4) The road was not wide enough to carry the required transport for the offensive.
All of these factors last year meant that the Austrians could have smashed any offensive coming from across the river. This is the "make a shambles" part.
This year, the author's side have managed to capture a "bridgehead" on the other side of the river.
bridgehead == a position held or to be gained on the enemy side of a river to cover the crossing of friendly troops.
Either the author's side doesn't need to use the road at all, or, some or all of the above factors have been dealt with. This year the offensive can go ahead.
Please feel free to comment if I'm wrong.