The phrase (not sentence) 'long-standing army' is grammatically correct. It means 'an army that has been established for a long time.'
There is a guru standing on my toe.
"They were as tired and depressed as a defeated army."
The boy was standing on the platform.
Officers carried swagger sticks.
Yes if you use it correctly, as in "i enllisted in the army"
He was wearing an army garb.
The army had a postwar celebration.
I was really shocked to see the bruin standing before me..
After the Nazis Army was incapacitated, they retreated.
The army marched down the hill.
He looked smart standing on parade wearing his black tunic :-)