The Durand Line, also known as the Afghanistan–Pakistan border, is a 2,640-kilometre (1,640 mi) international border between Afghanistan and Pakistan. India also claims a land border with Afghanistan on the eastern end of the Durand Line between Afghanistan's Wakhan Corridor and Gilgit administered by Pakistan by also claimed by India as part of the disputed Kashmir region The western end runs to the border with Iran and the eastern end to the border with China.

The Durand Line was established in 1893 as the international border between the Emirate of Afghanistan and the British Indian Empire by Mortimer Durand, a British diplomat of the Indian Civil Service, and Abdur Rahman Khan, the Emir of Afghanistan, to fix the limit of their respective spheres of influence and improve diplomatic relations and trade. Britain considered Afghanistan to be an independent state at the time, although they controlled its foreign affairs and diplomatic relations.

The single-page Agreement, dated 12 November 1893, contains seven short articles, including a commitment not to exercise interference beyond the Durand Line. A joint British-Afghan demarcation survey took place starting from 1894, covering some 800 miles (1,300 km) of the border. Established towards the end of the British–Russian "Great Game" rivalry, the resulting line established Afghanistan as a buffer zone between British and Russian interests in the region. The line granted Asmar and the valley up to Chanak to Afghanistan, while giving up claims to Waziristan and Chageh. It permitted arms imports from British India and increased the Afghan subsidy from 1.2 to 1.8 million rupees. The line, as slightly modified and ratified by the Anglo-Afghan Treaty of 1919, was inherited by Pakistan in 1947, following its independence.

The Durand line cuts through to demarcate Khyber Pakhtunkhwa, Balochistan, and the contested region of Gilgit-Baltistan of northern and western Pakistan from the northeastern and southern provinces of Afghanistan. From a geopolitical and geostrategic perspective, it has been described as one of the most dangerous borders in the world.

Although the Durand Line is internationally recognized as the western border of Pakistan, it remains unrecognized in Afghanistan. Sardar Mohammed Daoud Khan, former prime minister and president of Afghanistan, vigorously opposed the border and launched a propaganda war. However, Bhutto made a proposal in August 1976: that if he pardons the leaders of the National Awami Party then Daoud Khan must simultaneously announce that Afghanistan has no territorial claims on Pakistan and is prepared to recognise the Durand Line. Bhutto, however, also disclosed to American diplomats that he feared that the Afghans may go back on their word. Following the second round of talks, Daoud publicly announced his willingness to recognise the border as part of the Afghan concession, however, the following negotiations were stalled after Bhutto was overthrown a year later and following the overthrow of Daoud Khan in 1978, the new PDPA government back to the hardline stance on the Durand Line claiming that Afghanistan's borders extended up until the Indus River, essentially claiming more than half of Pakistan.

The area through which the Durand Line runs has been inhabited by the indigenous Pashtuns since ancient times, at least since 500 BC. The Greek historian Herodotus mentioned a people called Pactyans living in and around Arachosia as early as the 1st millennium BC. The Baloch tribes inhabit the southern end of the line, which runs in the Balochistan region that separates the ethnic Baloch people.

Arab Muslims conquered the area in the 7th century and introduced Islam to the Pashtuns. It is believed that some of the early Arabs also settled among the Pashtuns in the Sulaiman Mountains. These Pashtuns were historically known as "Afghans" and are believed to be mentioned by that name in Arabic chronicles as early as the 10th century. The Pashtun area (known today as the "Pashtunistan" region) fell within the Ghaznavid Empire in the 10th century followed by the Ghurids, Timurids, Mughals, Hotakis, by the Durranis, and thereafter the Sikhs.

In 1839, during the First Anglo-Afghan War, British-led Indian forces invaded Afghanistan and initiated a war with the Afghan rulers. Two years later, in 1842, the British were defeated and the war ended. The British again invaded Afghanistan in 1878, during the Second Anglo-Afghan War. The British decided to accept a new Amir who was a British opponent – Abdur Rahman Khan and the Treaty of Gandamak was signed in 1880. Afghanistan ceded control of various frontier areas to India. The British failed in their objective to maintain a British resident in Kabul but having attained their other geopolitical objectives, the British withdrew.