Verses 14-15 record Joshua’s instructions for the 2.5 tribes to help their brothers.  It’s probably significant that this is Joshua’s first take-charge moment.  The plot drives the theology of this passage.  Enjoying God’s rest is a family affair, not every man for himself.  God describes entrance into the Promised Land as “rest.”  It’s one angle on how life in the Promised Land relates to life in Christ.   Notice that every able-bodied warrior is needed in this fight.  Although the 2.5 tribes were already experiencing their rest, they couldn’t rest while their brothers and sisters had battles to fight.  The response of the 2.5 tribes urges us to obey our Lord.  They were so serious that they issued the death penalty on anyone who disobeyed.  Of course, on the cross, Jesus, the Obedient One, died in place of the disobedient ones.  By faith in Him we are able to obey and join the fight for the rest He provides (cf. Hebrews 4).