Hail the Ultramarines - swiped from Dakka

Circa M29: The Unification Wars. The Emperor of Mankind unites Terra, and sets off to conquer the stars. The Ultramarines’ Primarch, Robute Guilliman, is conveniently located on the other side of the galaxy when this occurs.

012.M31-014.M31: The Horus Heresy. Half of all Space Marine chapters betray the Emperor and turn to Chaos. Terra is sieged by the traitors, most Astartes legions suffer major losses, the Blood Angels, Imperial Fists, and White Scars are forced to make a stand on Terra, while the Dark Angels lose their homeworld,the Salamanders, Iron Hands, and Raven Guard are nearly destroyed,and the Emperor is crippled. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

546.M32: 'The Beheading': The High Lords of Terra are all slain on the orders of Drakan Vangorich, Grand Master of the Officio Assassinorum. The rogue Master of Assassins is tracked down and slain by a Space Marine strikeforce drawn from the Imperial Fists, Halo Brethren and Sable Swords. Only a single Space Marine survives the campaign. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

843.M35: Distress call from Grendel's World investigated. Planet discovered to have been attacked and all inhabitants slain by terror tactics of the Night Lords. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

Early M36: The Age of Apostasy and the Reign of Blood begin. Warp storms increase dramatically and Chaos and Ork attacks multiply. The Imperium falls into turmoil. An empowered Ecclesiarchy becomes more tyrannical. High Lord Goge Vandire becomes the most powerful member of the Senate of the High Lords of Terra, and manipulates a small sect, the Daughters of the Emperor, into becoming his personal bodyguards, the Brides of the Emperor. Vandire rules largely unopposed and more and more bloody and tyrannical. Zhoros (Fire Hawks' world) thermal bombed. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

266.M36: Sebastian Thor is born on Dimmamar. He eventually becomes a threat to Vandire, who sends the Frateris Templar fleet to destroy Dimmamar. The fleet is completly destroyed by a warp storm, still existing today, named the Storm of the Emperor's Wrath in the Clax system. Vandire's armies are finally defeated by combined forces of the Tech Guard and several chapters of Space Marines, organized under the banner of the Confederation of Light, led by Thor. Vandire is executed by Alicia Dominica, the leader of his own bodyguards, who reclaim their old name Daughters of the Emperor in a conspiracy involving the Adeptus Custodes and the Emperor himself. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

723.M36: The 5th Black Crusade begins. Doombreed, a prince of Khorne declares war on the Adeptus Astartes. The Warhawks and the Venerators are lost. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

995.M40-000.M41 The Macharian Crusades. Lord Solar Macharius sets out to reconquer the Segmentum Pacificus for the Imperium. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

444.M41: First War for Armageddon. Chaos incursion led by Angron and World Eaters beaten back by Space Wolves and Grey Knights. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

742.M41 Damocles Crusade is launched against the Tau Empire, a small, strategically insignificant piece of nothingness, inhabited by weak fish people with minimal Faster-Than-Light Technology. The Ultramarines are thoroughly defeated.

745.M41: First Tyrannic War. Hive Fleet Behemoth attacks the Ultramarines homeworld Macragge. Most of the chapter is eaten by Nids.

901-912.M41 Badab War. The Astral Claws, Lamenters, Executioners, and Mantis Warriors chapters rebel against the Imperium, and are defeated by loyalist chapters. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

941-942.M41: Second War for Armageddon. The Ultramarines bump into an Ork Waaagh! while running away from the Tyranids. They are saved by the timely intervention of Commissar Yarrick and the Blood Angels.

757998.M41 The Third War for Armageddon begins. Having learned from past mistakes, Imperial Commanders commit a ridiculous amount of forces to hold the world. It drags on for months in a bloody stalemate, and soon Ghazghkull grows bored and leaves his generals to finish the fight while he goes looking for other worlds to conquer. Commissar Yarrick joins a Black Templars Crusade and gives chase. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

5999.M41 Warmaster Abaddon launches the 13th Black Crusade. The armies of Chaos Undivided lay siege primarily to Cadia but attack many of the surrounding worlds as well. Imperial Forces launch a massive counterattack, and eventually push back the tides of Chaos, but only just barely, and at great cost. The Space Wolves' 13th Company are amongst the forces seen fighting against the forces of chaos. The Ultramarines are conveniently located on the other side of the galaxy when this occurs.

As an additional note of interest, there stands reasonable evidence that the Ultramarines are partially responsible for unleashing the Nightbringer on the galaxy in the current age. On the planet Pavonis, it was brought to the attention of Uriel Ventris, of the 4th company of the Ultramarines, that a group of revolutionaries were digging their way straight to the Nightbringer's sarcophogus while the Dark Eldar rounded up the keys to unlock the thing. Uriel was given a choice: he could either exterminatus the planet, burying the Nightbringer, revolutionaries, and Dark Eldar all at once in a single fell swoop, or he could boost his own ego by making a balls-first attack on the excavation site with nothing but chain swords and bolters. Unfortunately for the entire galaxy, Uriel chose the latter of these options, lost the fight, and let Nightbringer get free.