While I agree with you, tactically it has to be in phases. Would you care to list them?
IMO...the exit of US from Afghanistan can be used as an opportunity to convince US.
If US can be convinced then the western European countries, Japan, Australia and the likes won't pose a problem. China would be very easy to convince...and once US and China are onboard the "annexation" part wouldn't invite sanctions or any other retaliation.
US can be convinced by providing them assurances that Pakistan would take over the role of securing Afghanistan militarily...providing the same peace and stability that the western forces were supposed to do...and prevent Afghanistan from becoming a hotbed of ISIS and other such militant groups that would pose a threat to US. As a bonus "countering Iran" thing can be used. Iran would definitely try to gain influence after US leaves...and it would be a highly unfavorable outcome for the US. They would much rather prefer dealing with Pakistan than Iran...when it comes to Afghanistan.
For good measure...bribe US and China by throwing their companies billions in contracts. For US...throw some contracts for mining the mineral wealth of Afghanistan...and for China infrastructure development contracts and potentially even an extension of CPEC. This is not ideal...but it's just the way the world works. Plus knowing Trump...he would love that.
It would be even harder to convince the myriad of militant groups that make up Afghan Taliban...and other influential/powerful warlords, government officials, etc. to agree to a union with Pakistan. The current Afghan government and other elements of the northern alliance will definitely be against it. However they have little power on the ground...after the US exit. So if US ends up agreeing to that annexation than the current Afghan government/Northern Alliance would lose its only major supporter and would be easily discarded. In order to convince the Afghan Taliban and other key influential ppl...Pak will have to utilize its tribal links, religion, bribery, and it would be necessary to throw them a bone of political power in various forms.
^all that just to get the annexation part right in the eyes of the world(and Afghans themselves)...so Pak is not seen as an "invader" or "conqueror". We have seen what happens when armies march in to Afghanistan as conquerors...they just get bogged down.
Once that part is done...where it is a "union" rather than an "invasion"...then comes the challenge of first securing Afghanistan militarily. Afghan military would have to be demoted. I don't mean it in an offensive way...but rather more objectively...
...Afghan military is unprofessional...and can't be trusted. This however doesn't mean that they aren't useful. They would certainly have a treasure trove of information...like intelligence on the ongoings in their country, detailed maps, etc. Pak military with its main focus towards the east...doesn't have enough numbers to be deployed in a sizable manner for "peacekeeping" in much of this newly gained and much troubled territory. Certainly not all Afghans...and not all factions of militants would be happy with such a union with Pak...and would cause problems.
So perhaps...Afghan military can be used as a policing force...
...they would be initially downgraded from being the state army for reasons listed above(unprofessional and untrustworthy)...disbanding them outright would cause more joblessness and hatred(towards Pak) among them...which can be a disaster bcuz then there would be thousands of essentially militants who would want to return to the status quo(separate Afghanistan). So perhaps they can be made to work under or with FC?
This wouldn't be ideal...but it would buy time initially. PA can probably use its reserve force...to serve as the "superiors" in different areas of these FC type of Afghan army...carefully evaluating their training, performance, and loyalties. Over time...the Afghan Army personnel that fit the criteria in training(required in PA) and whose loyalties are with their new united country...can be recruited to PA...
...bcuz for an expanded country a bigger army would be needed. The ones that don't fit that criteria but are still loyal(to this new country) can be made part of the police force. The ones that are not loyal can just be either fired or punished...depending on if they have done something treasonous or not.
While all of those military challenges are being solved...simultaneously there would be enormous challenges on the economic front as well. It would take decades of hard work to bring economic prosperity to both Afg/Pak. Especially with this "union"...INITIALLY it would drain Pak's resources a lot more than Pak would gain from annexing Afghanistan. However in the long run I think it would be far more beneficial for both Afghanistan and Pakistan...and their ppl if both the countries united.
