CoA Kingdom of Poland (Rum)

Although stripped from power over Hungary by the Empire of Rum in 1526, the Jagiellonian dynasty had to ask the Empire for help, as it held desperatly to it's last parts of indpendence. In the years before, the Tsardom of Russia had repeatedly invaded Poland to secure buffer territory against the expanding Ottomans. It had seriously reduced Poland to a mere shadow of their former expense, and now threatened to rip it off it's independence completly. 

The Ottomans replied by sendig troops to Poland, positioning them at their eastern borders with Russia and it's western borders with the Holy Roman Empire, supporting the war effort against the Christian Alliance that ahd formed against Rum after it's invasion of Austria. Many historians consider this move to be one of the reasons the Alliance and Rum were able to quit waging war before toppeling Europe into all-out devestation. 

The Jagiellion's became vassals to the Ottoman Empire in the aftermath, but remained independent as a country thoughout it's history.

Credits: Elements taken from wikimedia commons.