Answered by Chan from China | Feb. 18, 2016 20:12
For all I know, there were four important combats.
The first was the war against Tujue, a nomad living outside the northern border at that time. The war started in 630 and lasted for over 30 brokenly.
The second one took place from 644 to 668 against Korea.
The battle against Arab burst in 751.
The last was between the central plain and Tubo, a tribe in Tibetan region, happened on and off during 623 to 907.