There were a variety of factors which influenced the development of Wu Shu (literally: War Art) or Kung-Fu in China (a process still unfolding as was demonstrated by a recent visit of a Kung-Fu Troupe from the Peoples Republic of China).
Certainly the fact that China is very old and had been fighting off invaders for quite some time contributes a great deal to the era Kung-Fu is thought to have taken substantial form in. The wars had a great deal of influence. Like the Romans, the Chinese put a lot of emphasis on Martial techniques and developed them to a remarkable extent. The seeds of contemporary martial arts stretch far back into China's history...