This means that he feels very drugged as if he had just taken an opioid or something. He hears a nightingale singing saying that the bird is spreading its happiness too much. He is also saying that he too is happy because he can hear the nightingale sing somewhere amidst the forest.
As a leader he should have at least considered the advice of
his men especially since they were on the side of caution. Also a good leader must be careful to weigh
the consequences of his actions before he acts.
It meant war with Britain and America wouldn't have the support of Britain like it did as a colony.