Laughing in the face of adversity or danger is really not all that unusual for soldiers in combat, and it's hardly unique to just US troops. I've seen similar behavior myself, many times, and it had nothing at all to do with drugs, other than the pure adrenalin rush you always get during combat, perhaps. It's more a cowboy sort of thing, if you know what I mean, like the famous southern "Rebel Yell".

Benzedrine isn't a "euphoric" anyway - it's just a very powerful stimulant - it's pure speed. You don't really feel like laughing when you're taking it. You just get super-intensely focused on whatever it is you're doing, and you seem to possess a virtually limitless energy.

More typical "symptoms" would be not eating, not sleeping, not shitting or pissing but once every 12-24 hours or so, and appearing to be wide-eyed and hyper-alert ALL the time.