On 15 September 1939, Stalin concluded a durable ceasefire with Japan, to take effect the following day (it would be upgraded to a non-aggression pact in April 1941). The day after that, 17 September, Soviet forces invaded Poland from the east. Although some fighting continued until 5 October, the two invading armies held at least one joint military parade on 25 September, and reinforced their non-military partnership with the German–Soviet Treaty of Friendship, Cooperation and Demarcation on 28 September. German and Soviet cooperation against Poland in 1939 has been described as co-belligerence.