What was the cause of the duar war

Britain sent a peace mission to Bhutan in early 1864, in the wake of the recent conclusion of a civil war there. After the British occupied Assam and made the area part of British India (1826), a longtime frontier dispute began with the state of Bhutan to the north.

Explanation:

However, on December 16, Shabdrung issued a proclamation, which accused the British of unprovoked aggression against Bhutan. He announced that the Bhutanese people should be ready to resist the attacks of the British and claimed that the British seemed determined to take away the freedom that the Bhutanese had enjoyed for centuries.

As British had captured these areas with some ease, they felt that they could disband their main force and left only one or two armed posts in the new territory and went for resting. Taking this advantage, Bhutan suddenly attacked and captured Dewangiri on January 29, 1865. They had already captured Bishensing, Buxa and Balla on January 25, 26 and 27, with the force led by Trongsa Penlop.

(from wikipedia, sum up your own words)