Can a Buddhist join the army?

Can the Buddhist lay person join the army of their own country? What is the Buddhist view on this? Is defending defenceless and protecting weak by joining to the military not allowed in Buddhism? And, what's the karmic (karma) difference of - non-military person hurting (in fighting) someone else vs - military person hurting (in fighting) someone else? So, if joining the army is not an option, **how should they defend their country?**