I just conquered Poland, with a Papal Invasion Casus Belli. "So what?", you might say. "What's so special about that?" Well, this invasion was special in all the right ways.
I started out as the House Nakonid of Lübeck, an independent count, for those of you who are not familiar with it, wedged between the HRE, Denmark, and the relatively powerful heathen Duke of Mecklenburg. Of Pommeranian and Slavic heritage, I immediately sought safety under the rule of my Polish cousins (figuratively speaking). With their help I pressed my paternal claim on Mecklenburg Duchy and won, only to find myself with a bunch of heathen counts, one or which is much more powerful than me, constantly plotting and causing problems for me, keeping my manpower drained. Well, I had time to deal with them before I moved on, I thought, and began to fortify my position. Then disaster struck. My character died, murdered on orders of his step mother, and the title passed to the half-grown daughter. Then, in the middle of yet another war with the heathen House Wizlavid, my spymaster tells me that the King of Poland, my dear friend, comrade at arms and teacher of my young brother, is plotting to revoke the county of Lübeck. I couldn't dissuade him, and I definitely could not win a war against him, but I definitely couldn't stand losing my main province and seat of power. So there was only one thing to do, I started plotting an escape route.
When the the demand of revocation finally came, I was ready. I turned it down, then immediately dispatched messengers to the Kaiser, hoping desperately that they would reach him before my own liege got the news. And to my fortune, they did. Before a formal declaration of war could be issued by the Polish King, I snatched myself out of his grip and declared my alliegance to the German Emperor, my de jure liege, and the Polack could do nothing but stand by in angry shame as I subverted both his power and plot.
The years went by. Neither of us forgot the other's betrayal as we raced to conquer the remaining pagans, he waiting for a chance to retake our lands, and I waiting for a chance for revenge and redemption. Eventually he died, but my grudge against him and his family. I had his daughter, now Queen, excommunicated. I watched as her countrymen rose up against her heretical rule, tearing her country it's seams, but t was not enough. So I petitioned the Pope yet again, giving my a glorious and holy Reason to exact my final revenge. And when the time was right, as she was trapped between two revolting vassals, I launched my invasion.
All her base are now belong to me. The Nakonids have taken everything from those who tried to take everything from us. Her line will die with her. I have avenged myself, and with a vengeance. This is the most glorious and noble of moments.
...Wait, what's that, Chancellor? You say the ex-Queen's cousin has just started a war for her claim on the throne?
...Damn blood feuds...