The Emperor is a symbol of strength. Its connection with Aries provides the inspiration for the blossoms of Spring. This also requires much battle, (a is ruled by E, see also the 2 of Wands, E in a). The rams butt together their heads in the same way that plants battle for access to the Sun. He is typically holding a “Globus Cruciger”.
This is another rare instance in the Rider Waite deck where the attribution is publicly divulged, with the Rams adorning his Throne.
Note in the Thoth Deck version, the flag held by the lamb. It is based on the proto-Hebrew version of the letter Tzaddi.