We present a classical direct-interaction theory in which the panicle variables are considered over an observer's past lightcone instead of Ihe usual consrant-time hyperplanes. Because a past lightcone is Loren=-invariant we avoid the usual problems of the simultaneity of the interaction for differently boosted observers. %e theory is cpvariant in the sellse that we show a two-particle realimion of the P o i n d group with interaction. We calculate the forces due to the interaction and find tensor force equations of type @ = F . U . For a Coulomb distance potential, the resulting force t e w r can be compared directly with Ihe Maxwell tensor as a special case.