By the summer of 1410, the war was a year old. What began as a revolt in Samogitia, ceded to the Teutonic Knights in 1404, exploded when Lithuania announced they would support their oppressed brothers. Poland, honoring the 1385 Union of Krewo, joined in. After an initial push into Greater Poland the Teutons were pushed back to the border and an armistice agreement lasting to late June allowed both sides to prepare.
The Polish/Lithuanian alliance, backed by Masovia, pressed forward on June 30 to attack the Knights at Malbork. Teuton Grand Master Ulrich von Junginen responded by fortifying the river crossings and pulling his troops back. Wladyslaw Jagiello, King of Poland and former Grand Duke of Lithuania, responded by flanking the Knights and moving towards Marienburg, the capital.
On July 15, both armies met between Grunwald, Tannenberg and Ludwigsdorf. Polish heavy cavalry guarded their left, while Lithuanian light cavalry dominated the right along with Tatar skirmishers and Moldavians. The Teutons consisted mostly of heavy cavalry and infantry along with Western European mercenaries. While future propogandists would push the numbers of the two armies into the hundreds of thousands, it's generally believed the Polish/Lithuanian alliance had about 40,000 men, while the Teutons numbered 25-30,000.
A major push around midday by the Lithuanians was repulsed, and Grand Duke Vytautas began a controlled, possibly planned retreat. The Knights' reserve recklessly followed while von Junginen and the Poles desperately fought. At one point the Grand Master led a charge against one of the strongest Polish banners, or units, winning briefly before Jagiello could regain the upper hand.
By the time the Knights' reserve returned and regrouped, it was too late to press their advantage. Vytautas also returned, and despite the Teutons' techonological edge (including field artillery), sheer numbers weighed them down. The alliance flanked their enemies, and by nightfall as many as ten thousand of the initial thirty thousand knights lay dead. More would have perhaps been captured, but as the Poles and Lithuanians consisted primarily of peasants who had no part in any honorable ransom between nobles, there was much slaughter. The Poles did indeed ransom those they did manage to capture, at exorbitant rates that drained the Knights' resources. On that bloody day the Teutons lost their leadership and will to fight.
II. DIPLOMACY IS OPTIONAL
After the battle, Heinrich von Plauen led the Teutons. He wrote letters to the west, describing his struggle as a "war against evil pagans." One could almost argue that Lithuania qualified, if you assumed Orthodoxy counted, but this certainly wasn't true of the Poles. Nonetheless his bold rancor won much sentiment throughout Western Europe, but little support.
A number of cities tried to defect to Poland, including Danzig. Von Plauen slowly brought them back into line, often by force, earning him many enemies among the burghers. The Peace of Thorn, by which Lithuania regained Samogitia and Poland took the Dobriner Land, along with general rancor alienated the knights.
In short, Hochmeister von Plauen was what the Knights needed badly in 1410. He represented order and law. He didn't accept criticism. Bluntly, he put the disloyal middle class back in their place. However, his inability to bring people together and perhaps treat somewhat more gently with others resulted in his ouster in 1413.
III. PLAYING NICE
The new Grandmaster was Michael Kuchmeister von Sternberg. He could have been von Plauen's opposite. Where was one rough, the other knew how to be gentle. He spent the next five years quietly rebuilding the Order and reforging ties with the burghers. He improved ties with the German states and worked to ease fears by the Order of the Sword, an order within the Teutons, that the Teutons might try to revoke their autonomy. (Livonian Order starts as a vassal.) By Christmastide in 1418, he'd succeeded in restoring some semblanace of calm throughout Teuton lands. (Stability +2 entering game.)
By then, von Sternberg was looking for a new raison d'etre. Despite his predecessors claims, he understood the pagans of Eastern Europe were long gone. On the other hand, he certainly wasn't going to disband the order...