[Tuor of Gondolin]

There is another way to look at this. Rohan and Gondor came to each other's
aid several times. For example, in 2759 T.A. Gondor sent aid "both east and
west of the mountains." And politically, any rational leader of Gondor would
see that, irrespective of sentiment, Rohan would probably be next on the
list of an invader if Gondor was occupied. The Oath of Eorl then might be seen
as a useful reinforcement and rationalization of a national self-interest policy.