Why didn't Gondor respond more quickly?
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They were stretched thin:

>Eärnil was a wise man, and not arrogant, even if, as to most men in Gondor, the realm in Arthedain seemed a small thing, for all the lineage of its lords.

>He sent messages to Arvedui announcing that he received the crown of Gondor, according to the laws and the needs of the South-kingdom, "but I do not forget the loyalty of Arnor, nor deny our kinship, nor wish that the realms of Elendil should be estranged. I will send to your aid when you have need, so far as I am able."

>It was, however, long before Eärnil felt himself sufficiently secure to do as he promised.

><sup><sub>*Return of the King* Appendix A "Annals of the Kings and Rulers" I "The Númenórean Kings" *(iv) Gondor and the Heirs of Anárion*</sub></sup>

Gondor is just coming off of a bad century, with the Wainrider invasion right on the heels of a massive plague. Eärnil made his promise in T.A. 1945, shortly after he finally defeated the Wainriders, and Gondor wasn't ready to commit to a major operation.

Why didn't the Elves respond?
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This isn't clear from the text. It's entirely possible that the Elves didn't know the war was going so poorly until Eärnur arrived at the Grey Havens:

>[A]t last in the autumn of 1973 messages came to Gondor that Arthedain was in great straits, and that the Witch-king was preparing a last stroke against it. Then Eärnil sent his son Eärnur north with a fleet, as swiftly as he could, and with as great strength as he could spare. Too late. Before Eärnur reached the havens of Lindon, the Witch-king had conquered Arthedain and Arvedui had perished.

><sup><sub>*Return of the King* Appendix A "Annals of the Kings and Rulers" *(iv) Gondor and the Heirs of Anárion*</sub></sup>

But we can't be sure.