Question

According to what agreement would the United States have to declare war against any nation that attacked Great Britain?

Answer 1

There were a couple of reasons. Britain was in the habit of seizing American ships and claiming that certain sailors were actually British deserters, who they would then press into service on their own ships. They also seized cargoes on the high seas, to prevent the cargoes from falling into French hands. They were deaf to American protests.
In addition there was a desire among New Englanders to annex Canada, which had a small population. It was thought that Canadians would be eager to join America. They weren't, and that campaign failed.