Old units made obsolete by new units you can't make

[DaveGold]

If you research the technology for tanks you can no longer build cavalry or lancers. This is true even if you do not have the technology for oil or any oil resources. In these circumstances you are actually penalized for researching technology as your military options are diminished.

I suspect similar problems may occur with other technology and resources, such as aluminium. The obvious solution is to make the technology for oil a requirement for the technology for tanks.