The Whitebeard War, officially known as the Summit War of Marineford, is one of the largest events within the One Piece series that drastically changes the balance of power in the world. This gigantic battle occurred in the naval headquarters of Marineford and was a result of the capture and the ensuing execution of Portgas D. Ace, the adopted son of Edward Newgate, otherwise known as Whitebeard.
Edward Newgate, captain of the Whitebeard Pirates, was considered "the strongest man in the world" and one of the four emperors ruling over the New World. He declared war on the world government to save Ace, resulting in one of the most epic battles in One Piece history. Whitebeard's overwhelming power was gained through his consumption of the Tremor Fruit, Gura Gura no Mi, that enabled him to produce powerful earthquakes and tsunamis.
The war finally began when Whitebeard's forces arrived at Marineford and began to engage the Marines head-on. There are a number of prominent characters introduced in this conflict, along with their powerful abilities. One such character is that of Admiral Akainu, who made a first major appearance during the course of this arc while mercilessly pursuing "Absolute Justice" and showing powers gained from consuming the Magma-Magma Fruit.
It is in this arc of the One Piece anime, in season 14, where episodes 457 to 489 introduce events of the Whitebeard's War. These episodes detail the frenzy and action of the white-bearded pirates and their allies with their matches against the Navy. The anime vividly details a jumbled field of battle with spectacular views of Devil Fruit powers, Haki techniques, and conventional warfare.
One of the most significant aspects concerning the Whitebeard War is its relation to Monkey D. Luffy's journey. He wasn't a member of Whitebeard's crew, yet he comes and joins the fight for his brother Ace. It shows just how much Luffy is determined and grown-up enough to hold himself against opponents way higher than him at that moment: the three Admirals Aokiji, Kizaru, and Akainu.
Several pivotal moments occur at the climax of the war that define the One Piece world forever. Whitebeard's last stand introduces the whole world to the existence of One Piece Treasure, thereby redefining the Great Pirate Era. This is a particularly poignant moment because Whitebeard was prepared to die in his attempt to save Ace but traded off his life just so he could have a chance to make a final statement to the world.
Another of the major events of the war is the death of Portgas D. Ace, in spite of all efforts put in by Whitebeard and his allies to save him. Thus, Ace's death at the hands of Admiral Akainu becomes a huge turning point in Luffy's character development and makes way for the ensuing time jump.
Another thing the Whitebeard War depicted was the deep politics within the One Piece world. It would include at least the Whitebeard Pirates and the Marines but extends to other major players: the Seven Warlords of the Sea, revolutionary figures like Monkey D. Dragon, and even late Pirate King Gol D. Roger through Ace's parentage and other flashbacks and revelations.
The outcome of the Whitebeard War had consequences that continued to be felt a long while after its end. It brought great changes into the balance of power between the major factions of the world, the rise of new dangers, and completely changed the public view toward the World Government and the pirates.