With the 37th Regiment of Foot he fought through the Flanders campaign, seeing action at the battles of Le Cateau, Tournay, and Boxtel.

Despite the regiment no longer existing, Erskine continued on full pay in it until 27 February 1796 when he exchanged places with his brother William, who was at the time lieutenant-colonel commanding the 15th Light Dragoons.

[1] With the army advancing along the coast towards Egmond aan Zee, the 15th were placed in a position to ambush any French force attacking the British artillery.

[1] Erskine was promoted to brevet colonel on 1 January 1800 and as a reward for his conduct at Alkmaar was appointed an aide de camp to George III on 25 December the same year.

[5] Erskine continued without a command until May when he was appointed to a newly created brigade of two cavalry regiments due to arrive shortly in Portugal.

The 1st Hussars of the King's German Legion landed on 25 May, but Erskine had to wait until June for the 23rd Light Dragoons to complete his force.

[4] Taking command of his brigade, Erskine's health began to deteriorate with an undiagnosed illness described at the time as a severe indisposition.

[7] Erskine's condition worried Wellington to the extent that the general sent a letter to him requesting that he return home before he declined further, and provided a carriage to take him back to Lisbon.

Wellington wrote again to Erskine, saying "I cannot conclude without expressing my concern to lose your assistance; I am convinced that, if you were to stay, you would be unable to afford me any, and that you will become worse instead of better".

[5] Back in Britain Erskine was returned to employment as a staff officer in 1811, holding a command within Sussex and the Western District.

[5] His brother William, who had inherited the baronetcy from their father in 1795, was commanding the 2nd Cavalry Division in Portugal as a local lieutenant-general.

[5][8] Erskine continued to serve in the Western District until he was promoted to lieutenant-general on 4 June, after which he was not employed again and saw no further active service.

[9] The art was purchased in order to decorate Torrie House; the National Gallery of Scotland has noted that while Erskine collected a number of very valuable works, as a group it is of very uneven quality.