Anon1mous- ok, so it is pikuach nefesh. Why does that mean if they decide to band together an army, for pikuach nefesh, that it is an army of klal yisrael that everyone needs to join? It is for pikuach nefesh alone, hilchos melchama do not apply to a case of pikuach nefesh.
And again, even if it would be a real halachic army, it would only be ages 20 to 60. How can you force them at 18 and still claim mitzvah?