The 25th Century is a span of 100 Earth years that started in 2401. The current year, 2411, is set in this century, which will end with the year 2500. So far, the 25th century is marked by large-scale conflicts among Alpha and Beta Quadrant powers, as well as many power struggles initiated by the Iconians and their many voluntary and involuntary servitor races.
This period covers the years immediately leading up to the start of the game.
- Councilor Konjah of the Klingon High Council is revealed to be an Undine after an attack by Lethean mercenaries. The members of his house are killed or forced into hiding.
- A contingent of retired and current Starfleet officers, including Ambassador Worf, Admirals Beverly Crusher, Kathryn Janeway and Chakotay and retired Captains Ezri Dax and Tuvok appealed to the Federation Council to reconsider its position on the Klingon-Gorn War.
- A group of Klingon pilgrims attempt to kidnap Miral Paris and take her to fill her role as the Klingon Kuva'magh.
- A group of Romulan officers opposed to the rule of Praetor Taris make contact with Sela and offer her their support for a rebellion.
- The U.S.S. Rapier disappears in the Bajoran Wormhole on a mission to the Gamma Quadrant. The Rapier was the first of a new class of Starfleet Escort and was fitted with many of Starfleet's newest systems and technologies.
- Klingon-Gorn War: Klingon forces attack and occupy Gornar. King Slathis surrenders and Klingon General Klag decides to treat the Gorn respectfully. After the arrival of Chancellor J'mpok one week later, parts of the Gorn leadership are exposed as Undine and publicly executed. The Gorn Hegemony is subsequently granted self-rule as part of the Klingon Empire.
- Sela leads a Romulan and Hirogen fleet to Rator III. When Praetor Taris is found to be missing, Sela assumes control of the Romulan Star Empire.
- The Klingons reinforced their claim to the Hromi Cluster, which was under Federation control. They demanded that all Federation assets, settlements and people be removed from the cluster within three months.
- J'mpok informs the Federation that he is invoking ancient claims to the Archanis Sector, and that all Federation citizens must leave.
- After their ultimatum elapsed, the Klingons invaded Korvat Colony in the Hromi Cluster and began to forcibly remove Federation civilians from planets in the cluster. Starfleet sends a fleet, establishing a blockade, and the war begins.
- The U.S.S. Stargazer-A makes first contact with the Xaraxian Congress, who were intrigued by the Federation as a political entity dedicated to peace.
- The Klingons invaded the Archanis Sector, a long-standing buffer between the Federation and the Klingons. This was a clear sign that the Federation-Klingon War was now official.
- The U.S.S. Enterprise-E investigates the loss of contact with Starbase 236, and is destroyed in a battle with the Undine. Captain Data and the majority of the crew are able to escape.
- Federation-Klingon War: Klingon and Gorn forces invade Cestus III and surrounding colonies, and Starfleet masses a large fleet to repel them.
- Sela is named Empress of the Romulan Star Empire.
- The Romulan Insurgency begins.
This period covers the main in-game years for Star Trek Online.
- Main article: 2409
- The Romulan Republic settles on New Romulus. Other rebellious movements in the Romulan Star Empire gain momentum and the secrets behind the Hobus supernova are discovered.
- The Borg start their first incursion into the Alpha and Beta Quadrants in more than 30 years. Later that year, a Borg invasion of Fluidic Space is brought to an end.
- Advances of the True Way movement and Mirror Universe forces are halted in Cardassian-Bajoran space. As is an invasion of Klingon space by the Fek'Ihri, an invasion of Deep Space Nine by rogue Dominion forces, and the invasion of Defera by the Breen.
- The involvement of the Iconians in many of the political turmoils of the preceding years is discovered by Starfleet and the KDF.
- An Iconian gateway is discovered on New Romulus. The gateway network is accidentally activated, leading to the discovery of the Solanae Dyson Sphere in the Delta Quadrant.
- Main article: 2410
- After a devastating Undine attack on both, Earth and Qo'noS, the Federation and the Klingon Empire declare an armistice, ending hostilities between the two powers. The Undine can later be convinced that it was the Iconians who deceived them into attacking the Alpha Quadrant powers in the first place.
- The Iconians make their presence officially known to the Federation, the Klingon Empire and the Romulan Republic and soon begin an all-out war against them. The conflict ends in the nick of time in Earth orbit, when the World Heart - which was previously retrieved with Krenim time travel technology - was traded for peace aboard the Iconian flagship.
- After a period of isolation, the Tzenkethi begin attacking remote systems in the Alpha Quadrant, using protomatter bombs to annihilate all life on the targeted worlds.
- The final resting place for survivors of the U.S.S. Enterprise-C is discovered at a defunct Tal Shiar facility in the Pictae System. The only living survivor is an alternate timeline version of Admiral T'nae, who is reintegrated with her prime timeline counterpart soon after
- Former Empress Sela is found and taken back into custody by the Alliance, and imprisoned at an undisclosed location.
- Long thought to be dead, Martok is freed from captivity in the Briar Patch. He returns to lead his house as a General, helping in the Alliance war effort against the Tzenkethi forces.
- The Nexus returns to the Alpha Quadrant.
- The target of the Tzenkethi Crusades is revealed to be the awakening of the Hur'q. With the help of the Dominion led by Odo, the Alliance manages to defeat and cure them on Empersa, costing Dukan'Rex his life. The Dominion is then incorporated into the Alliance, removing their addiction to Ketracel as a condition.
- Main article: 2411
- The Excalbians force members of the Alliance to participate in trials, inadvertently creating a simulated Borg threat.
- J'Ula's entry into the 25th Century caused a rift within the Klingon Empire over its direction and J'mpok's leadership, leading to a renewed Klingon Civil War. The Khitomer Alliance is dragged into the infighting.
- J'mpok is exposed at his secret headquarters at Gorath and as a result, the Klingon Great Houses and the High Council are disbanded.
- A clone of Chancellor L'Rell is reborn after a trip to Gre'thor, and she unites Klingons in the defense of Boreth from a new Fek'Ihri attack.
- J'Ula duels J'mpok to the death in the Greath Hall during the final battle of Qo'noS and emerges victorious, but passes the Chancellorship to L'Rell instead.
- 25th century at Memory Alpha, the Star Trek Wiki.
- 25th century at Memory Beta, the non-canon Star Trek Wiki.
- Star Trek Online The Future Past, Pre-launch promotional video detailing ingame history