Cattle raids.

The books politely call it cattle-raiding. Let's call it what it really was: cattle-rustling.

J. P. Mallory writes:
 

We have already seen how lexical correspondences permit us to reconstruct proto-Indo-European expression for 'to raid for cattle' and 'sacrifice of a hundred cattle'.  At first glance we might regard these as the chance residue of the vocabulary concerning the secular (raiding) and sacred (sacrifice) disposition of cattle in Proto-Indo-European society. But an extensive examination of the role of cattle in both society and believe among the Indo-Iranians, and a numbe of peoples in East Africa, Bruce Lincoln suggest the paramount role of cattle in early Indo-European economy and religion. [In Search of the Indo-Europeans ..., p. 137]

This is adapted from the diagram Mallory gives on p. 138:

1. The celestial sovereigns

2. give cattle to the *arya people who lose them to

3. the *dasa enemy, who steal them but are defeated by

4. the warrior classs who recover them in raids and deliver them to

5. the priestly class who sacrifice them to

1. the celestial sovereigns ---> 2 ---> 3 ...
 

Castor was killed in a cattle raid. Compare Apollodorus, Library and Epitome, 3:11:1 ff.

http://www.perseus.tufts.edu/cgi-bin/pt ext?lookup=Apollod%2e+3%2e11%2e2

There are some fascinating notes at the bottom of the page referenced.

The 10th Labor of Hercules also involves a cattle raid, where Herc grabs Geryon's cattle, and after many vicissitudes, gets them home.

This is just stuff I think about. I'm not going to make too many conclusions, but I prefer to think of the early IEs essentially as cowboys, riders rounding up cattle (either their own, or someone else's).

Mark.