If Pakistan gets involved in the war by siding with the Zionist alliance (Saudi Branch) and Iran retaliates, Pakistan's Western front will expose it's vulnerable Eastern front. India can easily take advantage.
Alternatively, the Saudis may understand Pakistan's predicament, and instead insist that Pakistan allow American forces to invade Iran via Balochistan and thereby free the Strait of Hormuz. If Iran loses its eastern most province, it may then capitulate.
That scenario is most likely in my opinion.
Saudis will use the Pakistani defense pact as a means to give the Americans the opportunity they desperately need.