After the French revolution of 1789, Britain, Holland and Spain sought to undermine the Republic, because they saw republicanism as a threat to their own monarchies. The French declared war on all three countries in February 1793. In response, Britain again increased the size of the standing army and in February 1794 the 21st Light Dragoons was raised for a third time.
Initially the regiment was employed in Manchester, providing aid to the civil powers, but eventually received its first overseas posting. In 1795 the French troops garrisoning the West Indies Island of San Domingo joined forces with the plantation slaves and declared the island a Republic. As a result the plantation owners requested British aid in quelling the uprising. Because the West Indies was crucial for British commerce, troops were dispatched to San Domingo. The 21st Light Dragoons was part of this force, though once there, the regiment suffered more from yellow fever than from enemy action. The British evacuated the island in April 1798.
In 1806 the 21st was deployed to Cape Town, to protect the Indian trade route. In fact the closest the regiment came to Napoleon and his armies was after the war ended in 1815, when the 21st furnished a troop to guard and escort the Emperor on the remote Atlantic island of St Helena.
Peace again saw a reduction of the army and, as one of the new regiments, the 21st Light Dragoons was selected for disbandment in 1820. The troop guarding Napoleon was allowed to continue until the Emperor’s death.