When would it become incumbent on us to actively participate in physical jihad?

Please can you assist me in understanding the ruling of jihad in todays age (where we are without a caliphate) and when it would become incumbent on us to actively participate in physical jihad? I ask because I am feeling increasingly helpless when I look at the atrocities that our brothers and sisters are enduring in so many parts of the world - e.g. Palestine, Syria, Lebanon, etc. I do realise that we can make duaa to Allah (subhanahuwataála), that He ease the affairs of our brothers and sisters, and that we can make donations and offer aid in relief work. However, are we not expected to do more, by way of uniting in physical jihad - e.g. in Palestine? At which point would this become obligatory for us? Are we waiting to unite under one caliphate before doing so? Please assist me insha Allah. I have tried understanding this issue for some time now. Unfortunately, I have not been able to find the sunni fiqh of jihad in todays age. I think that there are many people in confusion, and so they turn to less reputable on-line sources to understand their role in jihad better.