In the manual it is written that Armies cannot initiate battles by themselves:
An Army HQ with combat Units in reserve will react quickly to support Corps formations but should not be viewed as a combat Stack. It is important to note that a lone Army Stack will never initiate combat by itself.
For example, at the beginning of 1805 Russia can create two Armies in 1805, but no Corps. One is already created under Alexander, but I can create another. The reason is, what for if I cannot create Corps?