The helmet was re-awakened after so many men were injured in the head by shrapnel from overhead airbursts.
The French were the first to issue helmets, with the Adrian; modeled after a firefighter's helmet, it was very effective but expensive to produce. It is still used in ceremonial dress. Several armies, including Japan, used the Adrian helmet.
The Central Powers adopted the Stahlhelm. It was good for WWI but even better in WWII. It's design is still used, the current US kevlar helmets used many of its features.
The British used the Brodie helmet. It provided better protection for the head and shoulders than the other designs for troops in the trenches, but it was less effective in WWII and was replaced.
Producing troops with different helmets will provide infantry and crews for European and Asiatic armies.
The French and the Germans will certainly produce their own vehicles. Austria will probably follow Germany. The other nations are a bit of a toss-up.
Poland will probably follow the French model to spite Germany.
As near as I can tell from the fluff, Russia has collapsed. Part of the Ukraine may still survive, but the land that was Russia will be a battlefield. Some of the wealthy nobility and possibly Rasputin will be local warlords with mercenary troops and eclectic equipment.
The Ottoman Empire has faded, but they are still a power to recon with. They will probably eschew helmets for turbans. Because of the dearth of industry in the middle east, they will probably buy heavy equipment from across Europe.
I like the Second Draft of the Eagle, the more Baroque design. The more geometric designs are very attractive, but aside from being derivative of GW, they are artistically anachronistic. Art Deco is still over a decade away. Norman Bel Geddes was born in 1893 and will be 15 at the start of the war.