Even WWII was started on the assumption that it would be a quick and relatively painless war. I suspect that modern missile technology makes this assumption flatly invalid.
I always suspected that, with all the high-tech equipment, after the initial stockpile is gone (and a few hundred planes is not a lot if you look at WW2), it's much harder to scale up production (if one country alone even produces all the necessary parts like semiconductors, rocket engines, etc.).
US+EU+allies can probably manufacture all the war machines they need, as does China+allies.