Boundary disputes plagued Bhutanese–British relations. To reconcile their differences, Bhutan sent an emissary to Calcutta in 1787, and the British sent missions to Thimphu in 1815 and 1838. The 1815 mission was inconclusive. The 1838 mission offered a treaty providing for extradition of Bhutanese officials responsible for incursions into Assam, free and unrestricted commerce between India and Bhutan, and settlement of Bhutan's debt to the British. In an attempt to protect its independence, Bhutan rejected the British offer. Despite increasing internal disorder, Bhutan had maintained its control over a portion of the Assam Duars more or less since its reduction of Cooch Behar to a dependency in the 1760s. After the British gained control of Lower Assam in 1826, tension between the countries began to rise as Britain exerted its strength. Bhutanese payments of annual tribute to the British for the Assam Duars gradually fell into arrears. British demands for payment led to military incursions into Bhutan in 1834 and 1835, resulting in defeat for Bhutan's forces and a temporary loss of territory.