Does a nation’s foreign policy have to do only with matters of defense?

No

Sure, defense ranks high in the list of reasons for a nation's foreign policy to be the way that it is, but there are other factors, such as:

Economy: Trading alliances, Fundings for projects, economy, etc.

=> Trade: Trading for products that one's country does not produce (or in small quantities, depending on supply and demand).

This is just one of the many (excluding defense) that affect a nation's foreign policy.

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it was the first major victory for the colonists.