"Foe to ambition."
So we know a foe is an enemy.
An ambition is a desire to complete and do something.
Judging from this, the phrase may mean, "An enemy wants a desire to do something."
A counter argument is necessary, in order to show one's audience that one has taken into account the opposing opinion/thesis of the argument but still sides with their opinion/thesis; it allows the writer to once again prove their thesis.
