His first taste of land battle in Russia was the disastrous defeat at Narva, in which Lacy commanded a unit of musketeers, holding the rank of poruchik. During the Great Northern War he was seriously wounded on two occasions, also gaining the rank of colonel in 1706. In the same year Peter gave him command of the Polotskii regiment and three new regiments raising him to colonel status. The following year he led his brigade at Poltava, in which battle he greatly distinguished himself. In the battle of Rumna, 1708 he attacked and captured the headquarters of Charles XII. He gained fame at this stage by advising the Czar that musketeers should wait until they were within a few yards of the enemy before opening fire. Prior to this the Russians were known for uncoordinated fire. From this point began his fame as a soldier. His next active service, still under Prince Repnin, was the siege of Riga. Lacy was reputedly the first Russian officer to enter the capital of Livland and he was appointed the first Russian chatelain of Riga Castle in the aftermath.