Japanese fascists were a far bigger threat to the Soviet Union than the Nazis during the 1930s. Japan already sacked Nanjing. In one city, the Japanese fascists butchered the same amount of people as the Francoists did for the entire civil war. This is how brutal the Japanese fascists were relative to the Francoists. And unlike the Francoists, the Empire of Japan actually tried to invade the Soviet Union but got owned before venturing too far. Why should the Soviet Union send those 400+ tanks to Republican Spain without expecting something in return? If anything, the Soviet Union shouldn't have wasted material with a lost cause that is Republican Spain and shipped those tanks to the Republic of China instead where they wouldn't be squandered. One tank to Spain is one less tank to China. Frankly, Stalin bet on the wrong horse.