The Durand Line refers to a boundary that marks the borders of Afghanistan and Pakistan. Back at its formation, the line established in the Hindu Kush (a mountain range in southwest Asia) and named for Sir Mortimer Durand separated sovereignty of the region amongst Afghanistan, Balochistan, and British India. The line is 1,519 miles long.
The British ordered the demarcation with the Durand Treaty, and Afghan ruler Amir Abdur Rehman Khan
signed it in 1893. The division started over disputed land, and an attempt in 1949 by Afghanistan's "Grand Council" to void the Durand Treaty was ignored, as the treaty was such that both parties must agree and not make unilateral decisions about the division. Just two years prior, Afghanistan voted against Pakistan's admission into the United Nations, indicating high tensions between the neighboring countries. Some Afghan scholars say that after 100 years, the Durand Treaty was to expire and the disputed territory was to be returned to Afghanistan. Yet there is no mention of an expiration date in the Durand Line Agreement.
Controversy again rose when the Pashtun people, residing on the border between Afghanistan and Pakistan, were given only the option to join India or Pakistan. As of today, Pakistan's government urges the Afghans to recognized the Durand Line, though many Afghan politicians think it should be gotten rid of. Many Pashtun people in both countries also do not agree with the existence of the Durand Line.
Sources: Britannica, The Free Dictionary, Afghanistans.com, Afghanland.com, Wikipedia