Tell this to Kashmiris.
You probably think that Crimea should be part of Ukraine?
Yet NATO provided arms to terrorist groups and those arms then wound up in the hands of ISIS. Then when Russia exposed NATO's hypocrisy and succor only then did NATO changes her policy as she realised this way of regime change will not work in Syria like it did in Libya.
Some Kashmiris might not like being part of India, but they have no legal basis to claim that they should be part of
Pakistan, simply because there are no International Law that governs the situation.
There is no International Law that govern when and how to break up a country in parts.
There is no International Law that defines how to break up a colony.
Unions typically have such a clause. Both the Soviet Union and EU had/has such clauses.
The only legal event was the handover of Kashmir by its former ruler to India,
due to the violent behaviour of people wanting to join Pakistan.
The difference between Kashmir and Syria is that Kashmir is part of a democracy,
while Syria is a dictatorship with fake elections.
Crimea is one country overrunning another.
Syria started off as an uprising by the Syrian against the dictator,
but have since then been hi-jacked by Islamist terrorists.
The main reason for arms reaching ISIS is actually the lack of support from outside,
since undersupplied underpaid Syrians defected and/or sold the little equipment they got.
Already when the uprising started, there was debate whether there was any group within Syria
which would, if successful lead to a government interested in the wellbeing of Syrians,
rather than people just fighting for a right to oppress the rest.