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.
Not by the British Army, as they left Austria in 1955. The buildings are still standing and may be in use by the Austrian army.
Yes if you use it correctly, as in "i enllisted in the army"
The army had a postwar celebration.
He was wearing an army garb.
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.