AHC/PC/WI: Afghanistan Part of Iran

Can Iran have Afghanistan and be able to hold onto it somehow? I know Afghanistan and Iran are very similar (except for religion). But what would it require in order for Afghanistan to be absorbed into Iran without becoming a big white elephant?
 
This is basically impossible for a post-1900 POD, but pre-1900 Iran had a decent shot at it up to the early 19th century or so. The Safavids and Afsharids both controlled much of Afghanistan. Your best bet is a stable and powerful Persia going into the age of nationalism that panders to pan-Iranism to help control a region like Afghanistan.
 
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