Wodan... not sure what you're fishing for, but I'll play along...
So, WWI units: Assembly Line (AL)+Artillery
Subs: Electricity + Combustion
Yes... note you can very easily build subs before ever getting any WWI units, or conversely, you could build WWI units without ever getting subs... but yes, it's fairly flexible... when I refer to WWI-units, I generally refer to "Infantry, Dreadnoughts and WWI Tanks"... WWI planes have slightly differant requirements, but usually come-along the same time-frame (unless you're very focused on a particular research path).
So, to get WWI units AND subs, you need AL+Artillery+Electricity+Combustion, right?
Yep.
Which would = WWII units.
Nope... not at all... most WWII units require Artillery+Radio+Industrialism (WWII units being Marines, Battleships & WWII tanks)
Also, don't subs need oil? So you de facto need Combustion as well?
Yeah, Combustion is a pre-req for a ton of units, from Pre-Dreadnoughts and subs up to and including both WWI and WWII units... remember that Combustion unlocks oil production.
When I say "WWI units need this or that", I'm generally refering to what needs they have beyond subs... they DO require Combustion, but I thought that was a given, considering Kilroy's complaint that Subs were WWII units only, and not available at, during or before WWI units, which they in-fact, are.
Trust me folks... I spent a TON of time going over the tech requirements for during and between WWI and WWII.
Most WWI and WWII units actually require three or more techs... Early Tanks and Dreadnoughts for instance, require Combustion, Assembly Line and Artillery... WWI planes require Flight (which itself requires Physics + Combustion)... Subs require only Combustion + Electricity... which could come before, after, or at the same time as WWI units. It all depends on what you research first.
Most WWII units require Artillery, Combustion, Radio and Industrialism... the latter two both coming down the tech tree from any WWI requirement... WWII aircraft require Air Superiority (after Flight) plus Radio.
I only have two units that I consider "between war" units, and that's Heavy Cruisers and Destroyers... but it is possible to get them at the same time as WWI unit if you put-off Assembly Line till last.... but WWII Battleships, Tanks, Aircraft and Marines all require the WWI techs that you simply MUST have before progressing to WWII... there's simply no-way of getting WWII tanks, battleships, marines or aircraft without first having access to WWI tanks dreadnoughts, infantry or aircraft first.
Guys... I went over this with a fine tooth comb... everyone who questions the tech tree isn't even looking at it.
