From the Jain perspective, joining the army is generally not encouraged, because military service involves himsa (violence), while Jain Dharma is centered on ahimsa.

• Monks and nuns: absolutely do not engage in such activity.
• Lay Jains (householders): should try to avoid professions that involve harming living beings. If someone is in a difficult situation, Jainism stresses minimum violence, restraint, and right intention.

So the simple answer is: ideally, a Jain should not choose army service, because it conflicts with the principle of ahimsa.