: Although Rohan itself was an independant polity Eomer may have
: taken over effective control of Enedwaith as Aragorn's vassal.

[snip]

Eomer is never referred to nor implied to be Aragorn's vassal. Enedwaith was no longer claimed by Gondor, and was thus open to Eomer's claim.

It is not necessary to suppose some sort of vassalage developed to justify Eomer's extending his realm to include the lands of a defeated enemy who had not been under Gondor's control.

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