Peace and justice for all
1410 A.D.: Word comes from across the sea that an empire has fallen.
Tired of hearing "word from across the sea," and wanting to cross the sea themselves, the Swedish navy is revolutionized with the introduction of ocean-worthy caravels. Fast, durable, and armed with cutting-edge gunpowder cannons, they set Sweden up as the world's dominant nautical power.
1415 A.D.: Germany offers a large ransom for peace with the Continental Alliance. Sweden's share, alone, includes all the nation's remaining gold, an annual tribute to the Swedish crown, and the surrender of resources including spices, cotton, and iron. The Alliance accepts, and Swedish forces withdraw from Salzburg.
Later that year, the Celts offer a declaration of friendship. Despite their being at war with the Greeks, the Swedes accept, hoping to end the conflict and strengthen the Continental Alliance. Austria makes a similar declaration soon after.
Just when it seemed like things were going to settle down again, we've created a stark divide. Greece is not going to be happy with me, nor with the Austrians, for declaring friendship with a civ they are at war with. Germany is almost a non-player at this point, which leaves myself, Austria, Denmark, and the Celts. Greece is way ahead of all of us on an individual level, but I do have a decisive tech lead, and I doubt they could break us if we all stood together.
1425 A.D.: Greece gives into pressure from Austria and Sweden, making peace with the Celts. The Celts take the opportunity to immediately declare war on Germany, sending forces to Salzburg to aid the Danes (who never accepted Germany's peace offering and have still been besieging the city.)
The Swedes found a Chivalric Order of Awesomeness. Under the new code of chivalry, they sign mutual defensive accords with the kings of Denmark and Austria. Greece is excluded from the accord for as long as they continue to oppose accepting the Celts into the Continental Alliance.
Chivalry would allow me to build Knights... except that I still don't have Horses. This whole continent is almost bereft of them. Denmark and Greece each have some, but neither seem willing to trade for them. At least it allows me to set up Defensive Pacts, which will come in handy.
1430 A.D.: Germany successfully sues for peace with the Celts and the Danes.
Later in the year, the Greeks finally declare friendship with the Celts, cementing the five-way Continental Alliance. At the Council of the Five Kings, defensive pacts are agreed upon between all five nations. It is the end of an era of war.