Final answer:
The Peninsular War was the conflict causing significant issues for Napoleon before he invaded Russia, draining his resources and occupying his best troops.
Step-by-step explanation:
The ongoing war that was already causing significant problems for Napoleon before he invaded Russia was the Peninsular War. This conflict, which took place from 1808 to 1814, involved fierce battles between the French forces under Napoleon and the allied Spanish and British forces. As Napoleon continued his quest for European domination, a large portion of his best troops were embroiled in the Peninsular War in Spain, struggling with guerilla warfare tactics and draining French military resources. Consequently, when he launched his ill-fated invasion of Russia in 1812, many of the soldiers in the so-called 'Grand Army' were inadequately trained recruits from non-French territories, significantly weakening his forces for the campaign against Russia. The Peninsular War was a constant drain on Napoleon's resources, earning the moniker of Napoleon's 'Spanish ulcer' due to its persistent drain on his empire.