Numbers 32:20

And Moses said unto them, If ye will do this thing, if ye will go armed before the LORD to war,

Moses permitted the tribes of Gad and Reuben to settle on the east side of the Jordan only on the condition that they first made homes ready for their families and then crossed over armed alongside their fellow Israelites until every tribe had received its inheritance. The arrangement required them to remain with the rest of the host through the conquest rather than withdrawing early to enjoy their own portion. The verse therefore establishes the duty to share in the burdens and conflicts of the whole community instead of seeking personal ease while others are still struggling. No one was to claim exemption from common hardship because his own interest was already secured. For Christians this means that all who love God's cause are called to take their place in its spiritual conflict rather than leaving the work to a few.