You didn't give a context for your question. Perhaps your question was in regard to the Revolutionary War, or perhaps in reference to the War of 1812. The other respondent's answer gives good thoughts on why the US was unprepared for the War of 1812.
Let me just say a bit about the victory of the newly declared United States over Britain in the Revolutionary War. You would not have thought the former colonies would have been able to overcome the might of the British empire then, but they did. Some of the factors that helped the United States win its independence were these:
1. Strongly motivated, fighting for their independence.
2. Use of an outstanding spy network to gather intelligence on British troop movements.
3. Alliance with France, which provided much funding as well as military support.
4. Use of innovative tactics of warfare, such as snipers.
5. George Washington's decision to inoculate the whole army against smallpox in 1777. Smallpox was a major killer of soldiers on both sides in the war. Washington's bold move to inoculate all his troops gave the colonists an advantage.
