They are free to try.
If a nation can't contain and control terrorism and insurgency within its borders and it is affecting the security of a neighboring country - they can expect help, whether they'd like it or not.
Let's use a hypothetical Russian-Georgian conflict as an example. If North Ossetia was in rebellion from Russia and the Russians secured North Ossetia - but South Ossetia was part of Georgia and Georgia couldn't keep South Ossetians from doing cross border raids and then returning to safety... I would have supported a Russian incursion into South Ossetia to do the job that Georgia was responsible for, but unable to accomplish.
ESPECIALLY if Russia had been giving freebies and military aid to Georgia for a decade for the expressed purpose of securing their borders.
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