The Madhesi people of Southern Nepal are culturally closer to UP/Biharis as opposed to the Mongoloid Nepalis of the hill areas. The region they inhabit was part of India until the British gifted it to Nepal as thanks for helping quell the 1857 rebellion: The Madhesi people are discriminated against and referred to derogatorily as "dhoti" by the hill people and have had many of there rights curtailed by the Nepali constitution. Many armed groups have sprung up in defence of the Terai people including the "Terai Tigers", "Terai Virus" and the largest of them, JTTM: https://en.m.wikipedia.org/wiki/Janatantrik_Terai_Mukti_Morcha 40% of Biharis have relatives in Madhesh/Terai and they speak Indian languages like Awadhi, Maithili and Bhojpuri. It is in India's interest to annex this region from Nepal as it was never there's to begin with. One of the regions for the BJP's failure in Bihar was the failure to assist the Madhesis during the Madhesh Andolan of 2015. Further passivity by the government could lead to wide discontent in UP and Bihar.