The Embargo Act of 1807 was a defensive reaction because it was taken in response to the actions of Britain and France.
Britain and France were fighting a war with one another. The US wanted to trade with both countries, but both countries wanted to prevent this trade. Because of this, both France and Britain were doing things like seizing American ships trading with the other country. It was in response to this that Jefferson sought the Embargo Act.
In this way, the act was a defensive reaction to the seizure of American ships by European countries.