What lead to the modernisation of Japanese army prior 1900?

Answer 1

I would say the Meiji Restoration of 1867...led to a new form of government, that emphasized not only modernization of the army, but the country as a whole.
Secondly, The Japanese were highly influened by learning through text books about foreigners such as the Dutch, English and eventually the Americans would have the most influence.
There choice to modernize helped them be successful in the Russo Japanese war of 1905 and the Sino Japanese war .

Answer 2

If you watched the movie 'The Last Samurai' it gives a pretty accurate account of how the Post Civil War veterans from the USA went to Japan to help train their armies in warfare. Prior to then, the Samurai served the emperor as his army.