1
general::
A successful beginning, as in You've got the shopping list done? that's half the battle. This expression is an abbreviation of an 18th-century proverb, "The first blow is half the battle."
American Heritage Idioms
2
general::
Phrase(s): half the battle
Fig. a significant part of an effort. • Getting through traffic to the airport was half the battle. The flight was nothing at all.
McGrawhill's American Idioms And Phrasal Verbs