Its not Indus but Hindukush mountains was considered as border of India from ancient times and the two passes Khyber and Bolan were the entry points.
So as far as history is concerned NWFP/KP is a legitimate part of Pakistan not Afghanistan.
Dear Sir,
With the greatest respect, what 'Sparten' has stated is exactly correct; the NWFP/KP portion became the boundary line only since 1893, when Henry Durand signed an agreement with the Emir of Afghanistan defining the Durand line as the boundary.
Prior to that date, Afghanistan existed with a fluid frontier, which sometimes included the Punjab and beyond, right up to the Ganges-Yamuna Doab, and sometimes shrank, for instance in the face of the expansion of the Lahore Durbar and its outstanding general, Hari Singh Nalwa.
The Hindu Kush being a natural boundary is due to a habit of thinking loosely of 'India' as being bounded by the peaks on the north and the west; unfortunately for this sloppy habit, the trans-Indus territories were
at least as often (I am being polite) under the rule of the ruling power in what we call Afghanistan today, as under any other rule this side of the mountains.
If I may be permitted to remind readers, these trans-montane rulers of the trans-Indus sections include the Achaemenids, the Alexandrian (Macedonian) Greeks, the Bactrian Greeks, the Saka (Scythians), the Parthians, the Kushanas, the Ephthalite Huns, then after a gap during the Gupta Empire, the Hindu-Shahi/Turk-Shahi kingdom (ruling both sides of the mountains), the Ghaznavids, the Ghorids, again a gap during the Delhi Sultanate, or parts of it, the Mongols (specifically the Il-Khanate), and finally the Afshar and Abdali Empires. Against that, we have the Maurya Empire, the Delhi Sultanate and the Mughal Empire, and Lahore under Ranjit Singh pushing the frontier back. This is not intended to be an accurate or comprehensive list.
It would be interesting to know under these circumstances what makes the Hindu Kush to be considered the
natural boundary of 'India', and by whom. Natural obstacle, yes; passages in a natural obstacle, yes; 'deemed' boundaries, a somewhat over-extended definition.
Today, no doubt, NWFP/KP is very much a part of Pakistan.
Sincerely,