Re: Invaders from Sub-continent
british caste appeasement. they made a list of “warrior castes and tribes” to gain the loyalty of those that didn’t rise against them in 1857. even certificates were awarded, so one could show off and board the railway trains in second class instead of third class as brave warriors. 
truth is, most jatts are farmers, most pashtoons are pastoral nomads. warriors don’t generate any worth for the economy. and without an economy there is nothing to defend.
as for invasions out of the subcontinent - if you look at the geography of the indian subcontinent, there is only one outlet/inlet - via the khyber pass region. there are no agricultural areas once out of this region. just pastoral land. the iranian plateau is dry and deserted. there is nothing for hundreds of miles worth conquering. the indus and ganges plains by contrast were the opposite. agriculture from millenia had enriched the lands and the cultures of these parts.