After Alexander defeated the last of the Achaemenid Empire's forces under Bessus and Spitamenes in 328 BC, he began a new campaign to further extend his empire towards India in 327 BC. After fortifying Bactria with 10,000 men, Alexander commenced his invasion of India through the Khyber Pass. Whilst possessing a much larger army, at the battle, an estimated 40,000 infantry and 5,000 cavalry crossed the river in time to engage the enemy. During this battle, Alexander suffered heavy losses compared to his earlier victories.