That's a loaded question if I ever heard one.
The first elf defense we see is when Frodo had been stabbed by the Morgul blade. The four Hobbits had taken a rest from their flight from the Black Riders on Weathertop, a bit lower than the ruins of the Dúnedain watchtower, Amon Sûl. Frodo was sleeping when he was awakened by talking and laughter. He rushed over to find Sam, Merry, and Pippin had lit a fire to cook their dinner. Frodo scolded them for exposing their position, but it was too late—the Black Riders had found them. In the skirmish, the Witch King stabbed Frodo and, although Strider managed to chase the riders off, Frodo was gravely wounded. If he did not receive help, he would become a shade.
Strider and Sam split up to find athelas leaves to slow the poison, but the ranger didn't have much hope. As Strider bent down to cut off some leaves, a sword blade slipped under his throat. He prepared to strike, but when he looked up, he saw the elf Arwen, daughter of Lord Elrond and Granddaughter of Galadriel. She convinced Strider to let her take Frodo on horseback to her father. What followed was a hard ride as she and Frodo fled the Ringwraiths who chased so close at times they could almost grasp the poor Hobbit. Finally, Arwen reached the river ford. She called on her father's magic to turn the river to a herd of watery chargers that washed the Black Riders and their mounts toward the sea. Just in time, Arwen brought Frodo to Rivendell where Elrond healed him. Not completely, but enough to complete his mission of destroying the ring.
The problem with this account is that it's a fabrication.
In reality, Strider led the Hobbits into making the fire, knowing the Ringwraiths would avoid the flames. Strider did not wander off—he stayed and entertained the Hobbits with stories of Lúthien and Beren. The Hobbits didn't fight; they could barely see in the dark. Frodo did get stabbed, and Strider did chase the wraiths away. Frodo's wound was bad, but he didn't begin to turn immediately. In fact, he was able to make the seven-day journey to the bridge over Bruinen by riding Bill. Days later, after finding an abandoned troll cave guarded by the stone statues of the trolls Bilbo had once met, an elven rider appeared.
But it was not Arwen. It was Glorfindel. And Glorfindel did not carry Frodo away. The elf put Frodo on his horse, and Frodo stayed with the group who continued on foot. Several days later, the Black Riders caught up. Glorfindel yelled for Frodo to ride on, but the ring held him back. Finally, Glorfindel ordered the horse. At last, the horse brought Frodo across the ford where Frodo tried to order the Ringwraiths to leave. The riders began to cross, but the river, of its own accord, turned to white warriors on white chargers and swept the wraiths away.
So, was Peter Jackson wrong by changing the story?
I humbly suggest he was not. It was good for Arwen to have a more active role in the narrative than wandering around looking beautiful and concerned. Watching the party tromp through the woods for two weeks while Frodo grew progressively sicker would have slowed the pace too much. And the whole metaphor of Arwen bringing the heavily wounded Frodo to the powerful and caring hands of her father is too rich to wish otherwise.
Later, we come to another example of elf defense. One I'm much less charitable toward. The appearance of Haldir at Helm's Deep is a crime only slightly less heinous than the unexplained omission of Tom Bombadil. A regiment of elves
did not come to the aid of the people of Rohan at Helm's Deep and they were
not completely slaughtered by Saruman's massive army of orcs and Uruk-hai! Haldir was most likely protecting Lothlórien from goblins out of Moria! And he wasn't a great military commander—he was a squad leader of border guards!
I don't even know what to tell you. Are you saying the proud men of Rohan would let the
elves place themselves in the front line? Yes, I know Éomer was off with the strongest warriors, but still! Or that
all the elves would die when many of the men survived?
Ugh.
And, no. We're not going to talk about Tauriel's defense of Kili.
The additions of elf defense in the movies is a mixed bag. As I mentioned, I like Arwen's role. That was pretty cool. I have no idea what Jackson was thinking bringing elves to Helm's Deep. The whole point was that the men of Rohan were able (barely) to defend their people from the orcs until Gandalf, Éomer, and the trees could arrive. The whole point was that the time of elves was fading and the age of men was dawning.
And the less said about the fabrication of a elven woodswoman defending a strangely attractive dwarf the better.
For the most part, Peter Jackson should have stuck with the books. The War of the Last Alliance was a long time ago. The elves had long ceased having any ownership in the affairs of men. In the Third Age, excepting Legolas, the elves fought hardest when fighting in
self-defense.