Complex sentence: In March 1808, along with a Franco-Belgian unit of approximately the same size, the unit was deployed to Denmark, with the two-fold objective of protecting that country, also an ally of Napoleon, and preparing for an invasion of Sweden.
Split into simpler sentences: In March 1808, the French deployed the unit, along with a Franco-Belgian unit of approximately the same size, to Denmark. The objective was two-fold: protecting Denmark, also an ally of Napoleon, and preparing for an invasion of Sweden.