Answer:
The answer is indeed the last sentence of the excerpt: We must, therefore, acquiesce in the necessity, which denounces our Separation, and hold them, as we hold the rest of mankind, Enemies in War, in Peace Friends.
Explanation:
This excerpt starts by stating that Britain's relationship with America has become unbearable. It claims that America's appeals to Britain's kindness and magnanimity have been ignored by the latter. It is thus the duty of America to declare itself a free country. The last sentence, however, brings a tone of benevolence - probably the same benevolence America expected, but never received - when it states there is no need for hostility. If Britain wants there to be war, then the two countries shall be enemies. If peace is to be had, then they shall be friends. It is that simple. America will not permanently view Britain as its rival - that will only happen if extremely necessary, but it is not wished for.