The BT-5 entered service in 1933 and, with gradual deliveries until 1935, equipped all armored cavalry brigades. The first active engagements came in Spain, in 1937, when a batch of 100 was shipped to the Republicans. They took part in the defense of Madrid. Some were captured later and saw service with the Nationalist Spanish state. Two brigades (6th and 11th Tank Brigade) were sent in the Far East, on the Sino-Russo-Mongolian border to face Japanese incursions of the time. They proved instrumental at Khalkin Gol, with many BT-7s, proving too fast for the Japanese AT teams and still deadly for any AVFs deployed. However, they proved vulnerable to "close quarter" Japanese teams armed with Molotov cocktails.
Not long after, they were deployed in eastern Poland. They also soldiered during the "Winter War" in Finland, proving ill-adapted for the task with their thin armor. Losses were appalling. Finnish troops used Molotov cocktails as well and quickly found a weak point where the engine was installed, prone to catch fire and explode when hit, as shown in reports. In 1941 there were still hundreds of BT-5s in service despite the type having been replaced by the BT-7. But hundreds were lost or abandoned, worn out, during the summer offensive, liquidating what was left of the model. Only the lack of spare parts prevented the use of surviving vehicles until later in the war.
More information on http://www.tanks-encyclopedia.com/ww2/soviet/soviet_BT-5.php
