Temporal Cold War

The Temporal Cold War was a conflict fought between several time traveling factions, each from different points in time, and each attempting to manipulate history for its own benefit, in violation of the Temporal Accords. Each of these factions opposed one another in their attempts to gain dominance over the timestream, and often used proxy powers to carry out their missions. They were opposed by a group of temporal agents from the 31st century, who attempted to protect the integrity of the timeline.

Temporal powers

 * Mysterious benefactor of the Suliban Cabal (28th century). Unable to travel through time, only able to communicate. Motives very mysterious: has tried to both assist and thwart Earth interests in the 22nd century. Usually opposing the Federation. Also opposes the Sphere Builders.
 * Na'kuhl (29th century). Led by Vosk, this race vehemently opposed the Temporal Accords because they believe time travel is something to be used by all races for self-improvement. Responsible for the Cold War temporarily igniting into open conflict.
 * Sphere Builders (21st and 22nd centuries, until 26th century in a possible timeline). Beings from a transdimensional realm able to examine alternate futures, but seem to have limited time travel abilities as well. Tried to destroy Earth to prevent the Federation from ever existing, to stop their race's defeat at the Battle of Procyon V.
 * United Federation of Planets (31st century). Represented to Enterprise by temporal agent Daniels, who claims the Suliban Cabal's benefactor violated the Temporal Accords. Has acted against Cabal interests on several occasions. Also opposes the Sphere Builders and the Na'kuhl.

Proxy powers

 * Andorian Empire (22nd century). Assisted Earth during the Xindi conflict.
 * United Earth (22nd century). See "Enterprise and the Temporal Cold War" below.
 * Klingon Empire (22nd century). The Suliban Cabal tried to destabilize them and start a civil war.
 * Nazi Germany (20th century). The Na'kuhl provided an alternate timeline version of the Nazis with advanced technology in exchange for resources and assistance.
 * Suliban Cabal (22nd century). Performs actions for their mysterious benefactor in exchange for technology. Led by Silik.
 * Suliban renegades (22nd century). Former members of the Cabal, such as Sarin, who have left and now oppose its actions.  Assisted both Earth and the Klingons during the Broken Bow Incident.
 * Tandar Prime (22nd century). The Tandarans have been at war with the Suliban Cabal since 2144, aware of the Temporal Cold War, but their degree of involvement is unknown.
 * Tholian Assembly (22nd century). Tried to acquire a future timeship.  Alliances with any temporal powers unknown, though they seem to be opposed to the Suliban Cabal.
 * United Federation of Planets (26th century). Battled the Sphere Builders and defeated them at the Battle of Procyon V in a timeline now likely defunct.
 * Xindi (21st and 22nd centuries). Manipulated by the Sphere Builders into attacking Earth.

Enterprise and the Temporal Cold War
The had frequent involvement in different aspects of the Temporal Cold War, and indeed, its captain was responsible for its discovery by Earth.

Starfleet first became aware of the Temporal Cold War in April of 2151, when a Klingon courier named Klaang crashed in Broken Bow, Oklahoma. Spurred by a desire for humanity to display an ability to manage its own affairs, Enterprise captain Jonathan Archer elected to push up the launch of Enterprise by three weeks in order to return Klaang to his people. During the course of this mission, the Enterprise crew encountered a group of Suliban on Rigel X, who were actively opposing the Suliban Cabal, a group which was taking orders from a faction in the 28th century. Through Sarin, one of the leaders of the resistance group, Archer learned of the Temporal Cold War and of the Cabal's attempts to destabilize the Klingon Empire. When Enterprise returned Klaang to Qo'noS, the Klingon High Council was made aware that the Cabal had been staging attacks within the Empire, trying to make it appear as if one house were attacking another. 

That September, the Enterprise crew became aware of another faction in the Cold War, represented by Captain Archer's own steward, "Crewman" Daniels. Daniels revealed that he was from the 31st century, and that he worked for a group of temporal agents who were actively opposing the other factions and attempting to maintain the integrity of the timeline. Daniels enlisted Archer's help to locate Silik, a senior member of the Suliban Cabal, whom Daniels stated would be boarding Enterprise. Daniels was later apparently killed by Silik, who attempted to steal his temporal observatory, and deliver it to his mysterious benefactor from the 28th century. Archer managed to destroy the observatory, however, forcing Silik to leave empty-handed. 

The next year, the Cabal engineered the destruction of the Paraagan II mining colony, framing Enterprise for the disaster in an attempt to sabotage their mission. Daniels reappeared, however, and aided the crew in retrieving proof that they were not responsible, thereby allowing Enterprise's mission to continue. 

Later that year, the Enterprise crew learned that the Tholians were in some way involved in the Cold War, when they attempted to steal a ship from the 31st century. Enterprise eventually became embroiled in a three-way power struggle between themselves, the Tholians, and the Cabal, each of whom wished to obtain the ship. This fight was ended when Commander Tucker managed to activate the ship's temporal beacon, allowing Daniels' faction to retrieve the vessel. 

The Delphic Expanse
In March of 2153, the Xindi launched a devastating attack on Earth which resulted in the deaths of seven million people. Afterwards, the Cabal's benefactor provided Captain Archer with intelligence indicating that the Xindi were being backed by another faction in the Cold War, and informed him that the Xindi could be located in the region of space known as the Delphic Expanse. Embarking on a mission to find the Xindi before they could launch another attack to destroy Earth, the Enterprise crew became aware that this faction was responsible for the construction of the Spheres in the Delphic Expanse, for the purpose of reconfiguring space to make it habitable for their species, as a prelude to invasion. Having the ability to examine alternate timelines, these "Sphere Builders" became aware that, in the 26th century, the forces of the United Federation of Planets would ultimately defeat the Sphere Builders and drive them back into their transdimensional realm. Knowing that the Federation would never come into being if Earth were to be destroyed, the Builders contacted the Xindi and informed them that, in the future, humanity would be responsible for their destruction, and that, to ensure the survival of their species, they must destroy humanity first. Eventually, Archer was able to convince the Xindi Council that they were being manipulated into destroying Earth by the Sphere Builders, so that the Builders could continue their invasion unopposed. Captain Archer and his crew were eventually able to destroy the Xindi weapon before it was used against Earth, and make peace with the Xindi. They also destroyed the Spheres, banishing the Sphere Builders back into their realm. (}
 * Main article: Xindi incident

The Cold War heats up
The end of the Temporal Cold War constituted a predestination paradox, in which no one event could be considered to be the beginning. The Cold War escalated into an all-out conflict in which various operatives stationed throughout the timeline were given orders to change history. One of these changes was the assasination of Vladimir Lenin in 1916. This changed history in that Russia never turned to Communism. Not considering Russia to be a threat, Adolf Hitler was able to focus his war efforts on the West, eventually engaging in a successful invasion of the East Coast of America. It was at this point that the most dangerous faction in the Cold War arrived: the Na'kuhl. Led by Vosk, a fanatic from the 29th century who believed that it was his innate right to use time travel to mold the timeline to his own wishes.

The Na'kuhl had just narrowly escaped capture by Daniels's faction by using a form of stealth time travel, but found themselves trapped in 1944 due to the one-way limitation of this technology. For this reason, the Na'kuhl allied themselves with the Nazis, providing them with advanced weapons with which to fight their war in exchange for materials and manpower to build a temporal conduit with which to return to the future. Their plan worked and, using the conduit, the Na'kuhl launched an attack on Daniels and his faction, igniting the very conflict which led them to escape to 1944 in the first place.

In a last-ditch effort to restore the timeline, Daniels transported Captain Archer and Enterprise to 1944, in the hopes that they could stop Vosk before he launched his attack on the 31st century. Due to the massive alterations to the timeline which had taken place, multiple paradoxes occurred, causing temporal anomalies to rip through the timestream. These anomalies severely injured Daniels, who died in Enterprise's sickbay, but not before informing the crew of the necessity of stopping Vosk.

In the meantime, the Suliban Cabal's benefactor decided to take advantage of the chaos, and ordered Silik to stowaway on Enterprise when Daniels sent them to 1944. Silik was given orders to steal the specifications of the temporal conduit the Na'kuhl were constructing so that his master could gain the ability to physically travel through time instead of just communicate through it. Silik was eventually discovered and imprisoned in Enterprise's brig. Deciding that they had a common enemy in Vosk, Archer and Silik teamed up to stop him. Infiltrating Vosk's compound, they were successful in deactivating the shields around it, although Silik was killed in the process.

With the shields around Vosk's compound deactivated, Enterprise was able to move in and destroy it with photonic torpedoes, just as Vosk was about to enter the conduit, preventing him from initiating the conflict which led to his arrival in the first place. This had the effect of resetting the timeline and reversing Daniels's death. Bidding farewell to Captain Archer, Daniels informed him that the Temporal Cold War was coming to an end thanks to Archer's actions, and Daniels subsequently returned Archer and Enterprise to their proper place and time.

Background

 * According to Brannon Braga, the Temporal Cold War arc was created at the request of the studio, which wanted something more "futuristic". While Braga called it a "nifty idea", he later admitted that it "probably would have worked better as a separate show." (Star Trek Time Travel: Temporal Cold Wars and Beyond, ENT Season 1 DVD special features)
 * Rick Berman commented, "The idea that in the distant future...that time travel became possible but there were very very arduous rules about who could do it and what you could do, but there were people who were breaking the rules and there were people who were messing around, and there were other groups who were kind of watching over, and if you combine that with a variety of alternate timelines you can have a ball...Because you can deal with changing things that can be historical changes that can immediately be undone by resetting things. It gives us a lot to play with." (Star Trek Time Travel: Temporal Cold Wars and Beyond, ENT Season 1 DVD special features)
 * Regarding the end of the arc, Manny Coto remarked, "I felt that everything that had been said about the Temporal Cold War had already been said. I felt a heavy reliance on time travel at the beginning of Enterprise. I wanted season four to be a relatively time travel free season and that's why I debated writing it into season four."
 * Actor John Billingsley recalled, "I definitely felt as if there was a dictate on high from the network level, or from the studio level, to end the temporal time war, wrap it up immediately. I tended to concur on the broader point that the temporal time war never really got off the ground, the storytelling was too attenuated, and that it needed to die. At the same time I think the network forced them to tie it all up so abruptly that the way in which they had to do it was not as deft as it needed to be."

Apocrypha
The novel Watching the Clock explains factions involved in the Temporal Cold War avoid incursions in the time frame between the birth of the Federation in the 22nd Century and the destruction of the Borg in 2381. Previous novels explained that the Federation was indirectly responsible for both the creation and eventual destruction of the Borg Collective. Intervening in this time frame was seen as too risky for any faction because in every potential timeline where the Federation cannot destroy the Borg in 2381, the Collective assimilates the entire galaxy by the 27th century. The novel also establishes the motivates of the Cabal's benefactor as being attempting to eliminate the ancestors of key temporal scientists who would attend an important conference in 2381 that would lead to the construction of a subspace defense grid that would alert anyone to attempts from future time travelers to interfere with history, thus hindering attempts to alter the timeline.