I was playing with the soviets and was planning on conquering the world, but after I fought one turn and took one city the enemy suddenly became very powerful. It turned out all units suddenly get a bonus of +3449 combat strength. Even the levies now demolish my armies. I tried waiting a few turns and reloading but to no avail. He beat one of my small armies which had higher tech than him, maybe that's why he gets a bonus?