Apparently an ardent Orangist he joined the "Osnabrück Assembly," a group of former soldiers around Prince Frederick of Orange-Nassau who wanted to stage a raid into the Batavian Republic in the summer of 1795. After this project came to nothing he travelled to England where he became involved in the preparations of the Anglo-Russian invasion of Holland of 1799, which he joined on the British side. After the expedition came to nothing he joined the King's Dutch Brigade, a legion in British service founded by, and under command of, the Hereditary Prince. This regiment was formed from former personnel of the Dutch States Army and deserters from the Batavian army on the Isle of Wight in October, 1799. In December 1800, the infantry were put aboard transports and shipped to Cove in Ireland. They joined various regiments that were posted to Ireland at this time in the expectation that the implementation of the Acts to unite the Kingdom of Ireland and Kingdom of Great Britain into the United Kingdom of Great Britain and Ireland with effect from 1 January 1801, could cause some social unrest. The Brigade was later deployed in the Channel Islands and on the Isle of Wight against possible French landings. It was dissolved in 1802 and Detmers was put on half-pay. It is not exactly known what he did between 1802 and 1813. He may have returned to the Netherlands.
Waterloo and after
In 1814 Detmers again entered Dutch service as a lieutenant-colonel. He was promoted to colonel the same year and put in command of the 1st Brigade of the 3rd Division. As such he took part in the Waterloo Campaign At the start of the Battle of Waterloo the Dutch Third Division was placed in reserve on the right wing of the Allied Army under general Lord Hill. When the French Imperial Guard undertook its famous assault on the Allied right wing toward the end of the day, and the British line was hard pressed, the Dutch Third Division was ordered forward at the initiative of general Chassé. The 4th Grenadiers of the French Middle Guard were severely attacked by the battery of horse-artillery of the Dutch division, under command of captain Krahmer de Bichin, but they kept advancing. While the British line faltered under this onslaught, general Chassé ordered Detmers to charge the French column with his brigade. This was to be a bayonet charge, as Chassé had a predilection for this type of manoeuvre. The Dutch troops advanced in a state of high excitement, cheering wildly and lifting their shakos on their bayonets, according to a British eye-witness, who had a good laugh. But the Dutch soldiers had the last laugh, as they got the vaunted veterans of the French guard on the run. As this happened at the same time the French suffered a number of other setbacks, this retreat is considered the "tipping point" of the battle: 'Wellington' gave the sign for a general advance of the Allied army after which Napoleon's army started to collapse Some have speculated that because of this feat of arms Wellington referred to Detmers, when he mentioned "... general Vanhope, commanding a Brigade of Infantry of the King of the Netherlands" honorably in his Dispatch of 19 June 1815 to Earl Bathurst This may be possible as there was not a single "general Vanhope" in the entire Dutch army, let alone anyone by that name that warranted a mention in dispatches. In any case, Detmers received a Knight's Cross Third Class in the Military Order of William for his exploit in 1815. On 24 August 1816 he was promoted to major-general and appointed Provincial Commander of the province of Zuid-Holland in the Dutch Army. He still was in that post when he died in 1825.
