‘Good-morning; good-morning!' the General said
When we met him last week on our way to the line.
Now the soldiers he smiled at are most of 'em dead,
And we're cursing his staff for incompetent swine.
‘He's a cheery old card,' grunted Harry to Jack
As they slogged up to Arras with rifle and pack.
But he did for them both by his plan of attack.
‘Harry and Jack’,  are standard throwaway names, which represent all the men. They grumble to each other about the General’s jovilaity, because he remains “cheery” in this joyless environment; the General clearly doesn’t understand the life a soldier. Or it could be interpreted that, in their naivety, they were impressed with him.
The General