What is realpolitik?
the concept that political decisions of foreign engagement have to be based on the best interests of all parties concerned
the concept that political decisions have to be made by consensus or new alliances should be sought out
the concept that political decision about foreign engagement have to be based on preserving the interests of your country above all
the concept that a single country, acting alone, has the most realistic chance of meeting its own needs

The concept that political decision about foreign engagement have to be based on preserving the interests of your country above all.

Realpolitik is a foreign policy ideology coming from Europe during the 19th century and is centered around nationalism. 

Realpolitik is a nation-first focused foreign policy in which alliances are determined on whether or not it benefits a nation. This concept came out of Germany during their time of nation building. It suggests that alliances are important but must be strategic and important to the country. It the foreign policy is not beneficial then practically it is not a good decision for the nation to engage.