This may sound like a strange one but give me time!
"You and whose army?" is a sarcastic, humorous response to someone who is speaking about a plan which you don't believe they have the capability to implement successfully. For example:
[A is a junior office clerk in a large multinational company. B is a colleague of A.]
A: The CEO is being really unfair. Recently, he ordered everyone to work an extra hour every day. I am going to speak to him about it and persuade him to change his mind.
B: Oh yes? You and whose army?
Literally, B is suggesting that A will need a lot of people (an army) to help him achieve this objective. B is wondering who is going to lend his/her army to A to help him.
Can you think of any situations where you could use the phrase "you and whose army?" naturally.
Good effort Natalia. The situation needs to change a little so that the mother makes it clear that she is forcing her son e.g.
"If you don't do your homework by 8pm, I will make you do it."
"Oh yeah? You and whose army (are going to make me do my homework.)
Thia--We can hardly defeat Nysa Al Gul as she is a maverick fighter.
Laura-I can defeat her in one shot.
Thia--Oo Really??You and whose army??
These type of sarcastic phrase present in almost every languages and In my native language also we use these type of phrase to mock the ability of others especially when someone sounds like an empty vessel.
Thank you @Michael for the lesson:)