Join Date: Aug 2009
Location: Portland, Oregon
This is how I would spin it...
The marines should be both honor-bound and humanitarian. Then the situation that led to the Tau involvement: their chapter-world was under attack from some powerful external force (Tyranids, Orks, whatever you want). This threat proved too powerful for the marines to defeat themselves. They requested aid from the Imperium, but no help was forthcoming. Desperate to maintain their oaths of protection over their homeworld, the marines turn to the Tau.
The Tau agree to assist the marines if the Chapter allows free reign for the Water Caste and human facilitators (human converts to the Greater Good) to enter their homeworld for 250 years. The marines, seeing little other course, accept the agreement.
After defeating the enemy force with Tau assistance, the marines fulfill their portion of the agreement and allow the Water Caste and their human allies to preach the Greater Good. The marines and the human natives are little off-put by having Xenos on their world, but the message of the Greater Good rings true in their ears.
After the 250 years are up, the Tau get ready to pack and leave and...
Here's where I came up with a couple different ideas: 1. The Chapter tells them that they would be interested in joining the Tau Empire. This is a little...fast, in my opinion.
The other one is a bit slower. The Chapter allows the Tau to remain, if the Tau find that acceptable. The Tau stick around. Eventually, a few more centuries pass by. All the marines who grew up without Tau influence have passed on one way or another (or interred into Dreadnoughts). The highest echelons of the Chapter have grown up with the Greater Good, and the Water Caste, seeing this opportunity, offer a place for the Chapter in the Tau Empire. There's a lot of debate, but the conclusion was clear to everyone: the Chapter would join the Tau.