During the War of the Fourth Coalition, L'Estocq and his chief of staff, Gerhard von Scharnhorst, commanded some 15,000 troops based at Thorn in December 1806 and at Freystadt in January 1807.

Leading the last operational unit in the Prussian army, L'Estocq was only able to bring eight battalions, twenty-eight squadrons, and two horse artillery batteries (estimated at 7,000-9,000 men) to the battle; the rest of his soldiers were defending against Ney.

[3] Upon the small Prussian contingent's arrival at Preußisch Eylau, Bennigsen wanted it split up to reinforce his weakened Russian troops.

Scharnhorst, however, advised L'Estocq to strike with his cavalry around the Russian lines at Davout's exhausted troops; the sudden attack threw the French into disarray.

Colmar Freiherr von der Goltz wrote, "it was at Eylau in 1807, and not the War of Liberation in 1813, that the old army vindicated itself before the tribunal of history".