Is armed Jihad obligatory or recommended?

I am confused about this: Some teachings of Islam are orders that are obligatory, example Pillars of Islam: Salat, Sawm, Hajj, Zakat. Some teachings of Islam are recommendations on which it is optional to act on: Tahaajud, Sadqah, Umrah, applying Perfume, initiating Salam. My question is what about armed Jihad? Is armed Jihad is obligatory or recommended or optional? What is the evidence for this?