Originally Posted by Khezar

Bro, it goes without saying that subject selection plays a key role in CSS exam.
International Relations:
IR has been slaughtered a couple of times, and it has even doomed aspirants who majored in it. It was my optional in 2 attempts, but I have replaced it with political science for my last attempt. I prepared the subject without consulting a single local book/guide, but it didn't work. When too many aspirants are opting for the same subject, increased competition, and low score, is a natural outcome. On the other hand, you can find a few aspirants who score very well in it, and that's the beauty of CSS. So, carefully consider this choice. If you feel like opting for it, don't forget to study Perspectives on International Relations by Henry R. Nau (available online). Do analyze each question from theoretical perspectives; doing otherwise will make it a paper of current affairs.

Indo-Pak History:
I have no first-hand experience of the subject, but you are advised to search the forum for relevant threads where aspirants have shared their experience regarding the subject. The subject is considered tough n lengthy.

P.S: 'Tough, difficult, scoring' etc. are relative terms and CSS is full of surprises. So, don't take my words for granted; make your own enlightened decision.