Attack The Laagers is a 
Zulu decision.
It shows up in the decision tab, once either 
Transvaal or 
Oranje takes the decision Die Groot Trek. It is allowed if Zulu has a relation with either Transvaal or Oranje below 100.
It gives Zulu 5 prestige at the cost of 5 infamy. At the top of that, all Boer POPs in Durban (the province of 
Natalia) is reduced by 40%.
Boer POPs only make up 1.6% of the province of Durban, so the loss of POPs are neglectible. Still, however, it is not a very good decision, since the 5 infamy is way better used to attack another nation which on top of prestige also gives more POPs, more land and research points.
Ingame Text
The white men, known amongst themselves as 'Boers', have begun migrating from the British Cape Colony to the north and east, beyond the waters of the Ncome. We have seen that they come, bringing with them their women and their children, and we suspect that that means, that they intend to stay, but this is our land.