The Battle Tank is the heavy unit of the Modern Era. To train this unit, a Battle Tank Factory is required.
This unit upgrades the Tank in basically everything. gaining 15 attack points, 5 defense points, 1 range point and 6 movement points.
The attack and defense bonuses against Fast and Ranged Units are also upgraded by 5 points on each aspect.
This unit is devastating against the Paratrooper and the Mechanized Infantry. It's biggest disadvantages are the Bazooka Team and the Mechanized Artillery.
History
Modern Tanks revolutionized warfare. It combines the shooting range of a stationary canon with the mobility of a tank. Additionally the well shielded cabin inside the tank provides a decent defense against infantry.
Against Same Age Units
Paratrooper
The Battle Tank completely dominates the Paratrooper due to its bonus of 55 attack and defense versus ranged units. Even when the Paratrooper hides in the woods, the Battle Tank typically has enough mobility to still attack on the next round.
Mechanized Infantry
The Battle Tank completely dominates the Mechanized Infantry or Champion due it's large bonus of 55 on attack and defense. Both units have good mobility, bu the BT has far more firepower.
Bazooka Team
The Battle Tank perform poorly against the Bazooka Team, due to the Bazooka's bonus against heavy units. On maps with restricted movement, the Battle Tank can sometimes stay out of range of the Bazooka Team.
Mechanized Artillery
The Battle Tank perform poorly against the Mechanized Artillery, due to the Art. having much greater range along with a bonus against heavy units. On plains maps with fast movement, this advantage is significant, but on swamp / forest / hills maps, the Art. has a large advantage.
Other Military Units
Legend: 
= Light Unit Class 
= Ranged Unit Class 
= Artillery Unit Class 
= Heavy Unit Class 
= Fast Unit Class