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Allowed?
It depends on the what that means.
Are they allowed to seize private assets in time of war? No, but that doesn't stop the government from passing some law.
Are they allowed to ask private citizens to provide assets in time of war? Most definitely! Until World War I, private citizens were more heavily armed than the military. In the war of 1812, if it wasn't for the first Cajun Navy, the British might have rethought the peace agreement.

So how does the military go about asking private citizens for assets? I was thinking that the military has been pretty silent on blockchain technology and cryptocurrency but that both could essentially benefit the communication and financial sectors of the military.
Soldiers often talk about tactile aspects of the career. I think what Americans need to hear is the real, raw story of war from a soldiers perspective. Think of the book slaughterhouse 5. Something along those lines. People just don't quite or can't relate to the mentality it takes to hold up in war.
I can only relate because I have been through severe psychosis which scientist say is mentally as intense as going to and surviving front lines combat. I still have nightmares where I wake up screaming from the things I saw and heard during the psychosis and have been in remission for about six months now.
At the psych ward my favorite people to talk to were the veterans that were there. They are easy to spot. They are assessing the room and sitauation all the time, wait until last to get into line to eat, and don't speak very many words. I always tried to befriend them but they sadly most of the time would not have me. Believe it's because I am to incautious and I literally can not perceive danger as part of the illness j have. I can't read facial expressions or situations very well so I just go with everyone's my friend and the situation will be really good outcome. Niave, maybe, but i found it better mentally to cope with than trying to solve the situation for what it really is.
Thank you for answering my questions.

They just ask.
Just like how a sheriff's office would ask people to help in a search and rescue.