Need Help With War Item

[help101]

Hello, im not sure if this is WW1, 2 or civil war any help would be greatly appreciated

This is a hallmark card that is signed, not sure if it is family that signed this or several guys from a platoon

[Wagriff]

Going by the Smokey the Bear hat, I'd say it's likely WWI. It's signed by the guy and his Company's buddies, of course. Nice card! William

[Thanatos]

I would say it's a tad later.
During WW1 Hallmark was known as Hall Brothers.They took up the brand name and crown "Hallmark" in 1928.All Hallmark cards post this date have 'Hallmark" printed on the backs.The name of the company was changed to Hallmark in 1954.
This style of hat (doughboy to me) and it's bash(shape) is still used to this day,US Gunnery Sgt's would be a good example.
I think the history of the Hallmark company tell's the story.