The Russians avoided Napoleon's objective of a decisive engagement and instead retreated deeper into Russia. A brief attempt at resistance was made at Smolensk in August; the Russians were defeated in a series of battles, and Napoleon resumed his advance. The Russians again avoided battle, although in a few cases this was only achieved because Napoleon uncharacteristically hesitated to attack when the opportunity arose. Owing to the Russian army's scorched earth tactics, the French found it increasingly difficult to forage food for themselves and their horses.
Source: Wikipedia