In this role he attacked French and Dutch colonies at the beginning of the Napoleonic Wars in 1803, capturing Saint Lucia, Tobago, Demerara, Essequibo, and Berbice.

[7] The American Revolutionary War having begun, in June 1777 Grinfield travelled out to America in command of a draft of guardsmen, returning to England in January the following year.

While his services in North America are not recorded in detail, Grinfield was present at the surrender after the Siege of Yorktown on 19 October, and as such military historian S. G. P. Ward suggests prior to this he fought at the Battle of Guilford Court House on 15 March.

In the following year the commanding officer of Grinfield's 1st Battalion, Major-General Gustavus Guydickens, was suspended from his position because of fifty-eight outstanding debts at the Court of King's Bench and a criminal charge of gross indecency.

Little had changed, however, when in March 1793 Grinfield and his battalion, 600 men strong, joined the Guards Brigade to fight in the French Revolutionary War.

[1][10][11] Grinfield was singled out for his "personal bravery and ability" while fighting at the siege of Valenciennes between May and July, and in early August was promoted to become lieutenant-colonel of his battalion.

[21][22] He was then promoted to lieutenant-general later in the year, and in January 1801 was given command of the Midland, or Inland, Military District, consisting of most of Buckinghamshire and Oxfordshire with his headquarters in Lichfield.

[14][23][24][25] Early in 1802 Grinfield was transferred to command the Eastern Military District, before on 5 June he was appointed Commander-in-Chief of the Forces in the Windward and Leeward Islands.

[28][29][30] He toured all the British West Indies islands and also those belonging to the French in order to understand what he would have to seize in wartime, and brought together supplies and troopships for his men.

[14][28][29][30] Working in cooperation with Commodore Samuel Hood, Commander-in-Chief Leeward Islands Station, Grinfield's force set sail from Barbados on 20 June.

[10][32][33] This attack was made by two columns of troops, commanded respectively by the brigadiers Picton and Robert Brereton, and after half an hour of fighting the fort was taken, with the British suffering 138 casualties.

[34] The island was commanded by Brigadier-General César Berthier, who was forced to surrender on 1 July after Grinfield made a quick advance on his capital Scarborough with two columns of soldiers.

[37][38][35] The capture of Tobago at this stage stopped French plans to reinforce it, which would have seen Berthier build a strong naval depot guarded by a garrison of 1,200 men.

[39] This was to ensure that his force was not too thinly spread to respond to any French counterattacks from Martinique or Guadeloupe, with Antigua having refused to create a militia to help defend itself.

He received his orders on 10 August to go and accept the surrender of the Dutch governors, and a week later the Batavian Republic joined with France against Britain, making Grinfield's path to capture them more simple.

On 19 September his force took control of Demerara and Essequibo without bloodshed, the local commanders having surrendered on board the 22-gun post ship HMS Heureux the day before.

[10][44] There a flag of truce was organised and a committee was received to surrender the island, but the Dutch garrison commander refused to capitulate without discussion with his officers.

What boding omens, on the western gale,In tearful sympathy, this isle assail?Why, sad, responsive, doe Britannia sigh?Has fate decreed a nation's downfall nigh?Ah!

But yet a generous people mournTheir Grinfield dead, from them and glory tornThe verdant laurels, to his eager grasp,Yield, not relent, his warlike brow to clasp.Long, vainly, death in battle's storm had triedTo pierce his gallant breast with crimson dyed:In vain oppos'd the thundering cannon's roarAnd glittering steel; he firmly trod the shore –His country's cause bore down the opposing host,"My Country, God, and King," his only boast.