Why did Mance Rayder unite the clans?

Question

The King-Beyond-the-Wall, Mance Rayder, united the clans of the wildlings to save them from the terrible fate that awaits them in the long winter.

In the fifth season of Game of Thrones, he claims that it is his only motivation for doing that. To save his people. And twice in this season it is mentioned that it took him twenty years or so.

But this seems odd, for two reasons:

1. The summer lasted for ten years, so the last winter was more than that, and probably not very long either (I want to say three years, but I'm not sure).

2. The first documented sight of the White Walkers was quite recently.

Presumably the Free Folk know about the Walkers for a longer time now, but 20 years seems to go through at least one winter where nothing happened. If something had happened, there would have been repercussions (like many people fleeing south, and the word eventually reaching beyond the wall).

How did Mance know to start uniting the clans 20 years ago?

(Some minor timeline additions I feel obligated to make after re-watching S01E04. Ser Alliser tells Jon and Sam that during last winter he was part of a ranging party beyond the wall because they heard Mance was planning to attack the wall; and they got stuck for six months. So at least as far as TV canon goes, Mance was already powerful during last winter which is at least a decade ago.)

Answer 1

How much time it took Mance to unite clans, according to the books from 10 to 12 years.

In the very first chapter of the AGOT, Bran.

It was the ninth year of summer, and the seventh of Bran’s life.

The man had been taken outside a small holdfast in the hills. Robb thought he was a wildling, his sword sworn to Mance Rayder, the King-beyond-the-Wall.

Bran was born in 290 AC, so the chapter takes place in 297-298 AC. By this time Mance is already known as King beyong the Wall.

ASOS, chapter 7, Jon.

Mance says that he has seen Jon twice. First time -

You were just a boy, and I was all in black, one of a dozen riding escort to old Lord Commander Qorgyle when he came down to see your father at Winterfell. I was walking the wall around the yard when I came on you and your brother Robb. It had snowed the night before, and the two of you had built a great mountain above the gate and were waiting for someone likely to pass underneath.

Jon was born in 283 AC. Jon playing in the yard and building mountain of snow, well that makes him at least 3 years old (even in fantasy world a boy has to be old enough for certain things). So this event takes place in 286-287.

Now simple math, Mance deserted in 286 and in 298 he's King beyond the Wall, so at worst it took 12 years to unite clans. Mance deserted in 287 and in 297 he's King beyond the Wall, so at best it took 10 years to unite clans.

Motives and why did Mance united clans, well that's the hardest part of your question. We don't have Mances pov in the books and I don't recall him saying what his motives are. So we have to rely on other characters.

AGOT, chapter 57, Bran.

Bran has conversation with Osha

“Are there truly giants beyond the Wall?” he asked Osha, uncertainly.

“Giants and worse than giants, Lordling. I tried to tell your brother when he asked his questions, him and your maester and that smiley boy Greyjoy. The cold winds are rising, and men go out from their fires and never come back … or if they do, they’re not men no more, but only wights, with blue eyes and cold black hands. Why do you think I run south with Stiv and Hali and the rest of them fools? Mance thinks he’ll fight, the brave sweet stubborn man, like the white walkers were no more than rangers, but what does he know? He can call himself King-beyond-the-Wall all he likes, but he’s still just another old black crow who flew down from the Shadow Tower. He’s never tasted winter. I was born up there, child, like my mother and her mother before her and her mother before her, born of the Free Folk. We remember.”

Actually this passage once more confirms that Mance has not spent 20 years uniting clans, as Osha points out - He’s never tasted winter. Mance has not spent winter beyond the wall. And if we believe in Oshas words, Mance is going to fight Others. So that's your motive, if you have to fight others and wights you need an army. As to how Mance knew about others, well he might not but as Osha says We (Free Folk) remember.

As to why free folk attacked the Wall, I do recall Jon saying that free folk was not attacking the wall because of feud with Night's Watch, but because they were fleeing from something more terrible than swords and arrows of Night's Watch. And Mance was looking for safe house for his wife and his unborn child. I'll look in the books for direct quote and will include it later.

Maybe 20 years is a TV canon, but that doesn't comply with the books.

Besides, Tormund is not a reliable character. If Tormund is trusted then we have to believe that he has bedded bear and the bear has bitten half of his member but his member is still thrice in size compared to Mance's. Also he has spent some time in the belly of a sleeping giant and when the same giant woke up, took Tormund for her babe and suckled him for 3 moons turn. (this last paragraph is not part of the answer, but couldn't resist myself of not pointing out Tormund's exploits)

Answer 2

There are a few possibilities:

The White Walker Invasion has been going on for longer than we know

As you've noted, the Wildlings have a better feel for the coming of the White Walkers than the Southerners do, since they live so far North. Perhaps even twenty years ago there was a perceivable increase in White Walker activity that gave Mance the idea to unite the clans.

Escaping the White Walkers is a new priority

It's quite possible that Mance began the act of uniting the clans for other reasons like creating a strong enough nation/alliance to defy the Night Watch (from the books we know that Mance was a former Watchman who came to disdain their constricting way of life), then when the threat of the White Walkers made itself manifest, the priorities changed to escaping south of the Wall and away from the threat. It's interesting to note that the "took twenty years to unite the clans" assertion is absent from the books.

Relocating the clans was a long-term goal

Perhaps when Mance first started uniting the clans the threat of the White Walkers was still not a reality, but Mance recognized that it may become a reality in the future. Or perhaps he recognized that the clans would prosper better away from the extremely harsh conditions north of the Wall. So his goal was indeed saving the clans, just not from the White Walkers.