The Resistance is a loose and covert connection of humans and Vortigaunts formed with the shared goal of stopping their oppressive arch-nemesis known as the Combine from the oppression under the order of tyrant advisers and restoring their freedom of humanity. It is also served as the main hero force of the Half-Life franchise, first appeared in Half-Life 2.
The Resistance was formed shortly after the Seven Hour War in response to the Combine takeover. A loose band of guerrillas, outlaws and spies dedicated to overthrowing the new world order, they were unable to take on the alien empire directly. So the Resistance instead worked at undermining it by infiltrating the ranks of Civil Protection, smuggling weaponry, setting up a sophisticated underground with bases at Ravenholm, Black Mesa East, and White Forest, and by helping civilians escape the Combine controlled cities.
Many of the Resistance's key members are Black Mesa Personnel who survived the Black Mesa Incident and the events that took place afterward. Other key members are former officers of pre-Seven Hour War national armies (like Colonel Odessa Cubbage) or seem to have become leaders due to useful knowledge or experience.
From its formation until the Uprising, the Resistance mostly operated underground, with safehouses, hidden bases and concealed supply caches in and around City 17. One of the few exceptions to this appears to have been the Coast, where the Resistance had sufficient strength to operate more openly, with a number bases and outposts, including Shorepoint Base, New Little Odessa, Bridge Point, and Lighthouse Point. Operating from these positions, they regularly sparred with the Combine forces in the area. Over time, though, the Combine made significant inroads into the Coast, destroying many of the smaller outposts and condensing the spread out Resistance forces into larger groups at the main bases. Bridge Point was taken over and occupied, possibly due to the significance of the railway bridge beside it, which the Combine Razor Trains use.
Also around this time, Ravenholm, a secluded mining town connected to Black Mesa East, became one of the main Resistance settlements in the City 17 region, and was home to many escapees from City 17. Upon being discovered by the Combine, the town was subjected to a massive Headcrab Shell bombardment with only one known survivor. After this disaster, the passage between Ravenholm and Black Mesa East was sealed off.
While some small strikes took place against the Combine, most of the advances that the Resistance made during this time were minor; although the support they enjoyed from the citizen population was considerable, the Resistance had not the capability to attack the Combine openly. This changed with the return of Gordon Freeman, who thanks to his actions at Black Mesa had become to be a figure of legend among the Resistance. Shortly after his arrival in City 17, Gordon made contact with Resistance members Isaac Kleiner, Barney Calhoun, and Alyx Vance. After the teleporter meant to take him to the main base Black Mesa East malfunctioned, he was instead sent on his way through the Underground Railroad network to Black Mesa East. Meanwhile, the Combine, who learned of Gordon's presence when he was accidentally teleported to Dr. Breen's office, launched a citywide sweep to find him, cracking down on the Underground Railroad stations and flooding the tunnels and canals with Manhacks. With many safehouses and stations destroyed or discovered during the crackdown, the railroad disintegrated and most of its members were either killed or captured.
Although Combine forces pursued him every step of the way, Gordon managed to escape and made it to Black Mesa East. But shortly after his arrival, it too was attacked and destroyed. Eli Vance and Judith Mossman were captured, while Gordon, Alyx Vance and Dog escaped.
Even despite these blows, the Resistance held together, and their hopes were gradually amplified by Freeman's feats against the Combine. The resistance has shown remarkable cohesion, and even though they are not above criticizing each other and leaders like Kleiner and Cubbage, so far no infighting or betrayal (except for the case of Judith Mossman) had been seen within its ranks.
After escaping through Ravenholm, Freeman helped fight off a Combine raid on Shorepoint Base base and made contact with Alyx Vance. They had learned that Eli Vance and Judith Mossman had been taken to Nova Prospekt, and Alyx resolved to go there and rescue him. Gordon set out along the Coast to reach Nova Prospekt, helping to fend off an attack on New Little Odessa along the way. Finally, he reached Lighthouse Point and joined its personnel in fighting off a determined Combine attack. Despite their efforts, the base was overwhelmed and presumably destroyed.
After reaching the Vortigaunt Camp, Freeman attacked Nova Prospekt with the help an army of Antlions and, once inside its train depot, met up with Alyx Vance, who slipped in by Razor Train. Fighting their way through the facility, they found both Eli and Judith, but discovered that Judith had betrayed the Resistance and had been working for the Combine. Alyx and Gordon captured Judith and rescued Eli, and intended to use the Combine's teleporter to teleport themselves to Dr. Kleiner's lab. Instead, Judith tricked them and teleported herself and Eli to the Citadel, leaving the two to hold out against scores of soldiers while the teleporter charged up the second time. They managed to teleport themselves to Dr. Kleiner's lab, destroying the teleporter and the entire facility in the process. The destruction of Nova Prospekt was the first major victory for the Resistance, and it was taken as a signal to begin the uprising. The Resistance, who had previously only operated underground, engaged the Combine forces in open warfare, with many citizens joining their ranks. This marked the start of the Uprising.
Lasting over a week, the Battle of City 17 was a costly Resistance victory. The Citadel, as well as City 17, were destroyed, cutting the Combine forces on Earth off from the rest of the empire. The remaining Resistance forces, having evacuated from City 17 before its destruction and regrouped at the White Forest base, began preparing for the after effects of the Citadel's destruction. The meltdown of the Citadel's core created a superportal, which the Combine would use to bring in huge reinforcements once it had grown large enough. To counter this threat, the Resistance prepared and launched a rocket carrying a satellite which, once in orbit, would close the superportal. The battle to protect the rocket during its launch was furious, but the Overwatch attack force was eventually beaten back by a small force led by Gordon Freeman. After the main force was defeated and as Gordon and Alyx Vance were preparing to leave for the Arctic, two Combine Advisors made a surprise attack, during which Eli Vance was killed.
Meanwhile, Judith Mossman had discovered the Borealis, which carried Aperture Science technology. A debate raged over whether to destroy the ship or use its technology, and it is unknown which opinion won. Mossman was cut off by Hunter and Elite attacks, and her escort was killed, so Freeman and Alyx will probably journey to the Borealis through use of a helicopter to rescue Mossman and decide what to do with the technology.
The Resistance has no central command structure, however, there are central figures. Eli Vance, Barney Calhoun, Arne Magnusson, Alyx Vance, Isaac Kleiner, and Gordon Freeman are examples of Resistance leaders. The Resistance is divided into two areas: Combat and Research. Scientists such as Eli Vance command the research division, whereas leaders such as Barney Calhoun and Gordon Freeman tend to command the combat section. Gordon Freeman is a man of very high regard among Resistance members, something of a personal hero to them, due to the fact that he was able to not only survive the Black Mesa Incident (which only six people are confirmed to have survived, and even fewer confirmed to have survived the Seven Hour War), but actually put an end to it.
The core Resistance army is divided into four sections: regular resistance troops, who are fully fledged resistance members, equipped with body armor and supplies (some are trained medics as well); the civilian militia forces, who are citizens who have just gained weapons; the refugees, who are tasked with aiding the escape of civilians and fighting the Civil Protection; and the Vortigaunts, a very intelligent Xen species, who are adept at combat, and are able to use the Vortessence to spy on Combine activities. The Resistance is also aided by citizens, who help the troops and refugees by leaving behind caches full of food, weapons, ammo, and medical supplies.
In the Combine controlled world, humanity readily utilizes stolen Combine technology. Aside from weapons and armor, the Resistance has been seen to utilize the Hopper Mine, the Turret, the Thumper, the Rollermine, and Dr. Breen's Public address system, among others. In order to distinguish converted Combine tech, they will often be sprayed with lambdas, or in the case of a turret, WWII style nose-cone shark teeth. Humanity also uses its existing technology well, scavenging and repairing radios, transmitters, receivers, T.V. Screens, cameras, computers, etc. Cars, watercraft, and structures are built by hand. Despite the Combine rule, the Resistance science teams have created weapons like the Magnusson Device and the Gravity Gun, and a teleporter smaller and more advanced than the Combine's teleporter at Nova Prospekt.
Perhaps the greatest asset to the Resistance are the Vortigaunts. The Vortigaunts, as stated above, are intelligent, wise and very well versed at combat. They are able to communicate with all Vortigaunts instantly, an asset that they use to spy on the Combine in such places like Nova Prospekt (Vorti-Cells were Combine made devices to stop Vortigaunt communication, but were cut from the game). They are seen preparing headcrab as food, and their history with Antlion husbandry could prove handy in the future. With cases like Alyx, they can bring a human back from the verge of death, as well as using their power to activate generators and charge powered items like televisions and Gordon's HEV suit.
The Resistance was an underground movement in City 17, where most actions were covert. Spies like Barney Calhoun infiltrated the Civil Protection by going undercover, in order to keep attention away from Dr. Kleiner's lab, assist civilians in escaping City 17, and possibly alerting people of inspections and raids. Outside of the city, however, the Resistance is able to make frequent offenses against the Combine, and provide itself with sufficient defense. The Resistance had taken over a vast portion of the Coast, taking major bases such as Lighthouse Point and New Little Odessa. The Resistance's lack of numbers in this area however meant that they were frequently under attack by Combine raiding parties, and they struggled to hold onto their gains.
The Resistance generally operates as a shadow movement similar to many terrorists or rebel organizations in the modern world; unable to take on the Combine Empire directly, the Resistance seeks to undermine it by strategic moves. The Resistance held several strong points in and around City 17, such as Kleiner's Lab and Black Mesa East, before both were destroyed by the Combine. Ravenholm was also a major Resistance stronghold, before a Headcrab Shell bombardment killed the entire population of the town, with only one known survivor, as seen in the game, the first resistance was at the apartment raid in "d1_trainstation_03" when Gordon was ascenting the second staircase and a citizen told him to get in and head for the roof when the apartment raid was about to start, and before the loading screen to "d1_trainstation_04" when Gordon entered in the room that was leading to the roof.
Citizens of City 17 who choose to escape to Black Mesa East, a very large resistance base, have a very hard time getting there. The only known path is through City 17's canal system, which is subject to frequent raids by the Civil Protection force hoping to capture or kill any unfortunate refugees, as well as Xen wildlife such as Barnacles, Headcrabs, and Zombies. This is how the Resistance Underground Railroad was formed.
Resistance members created the Underground, a series of safehouses that help Resistance soldiers and Citizens escape to Black Mesa East, using coordinated safehouses and radios, and an Airboat to lend to refugees. Unfortunately, while Gordon Freeman was moving through the Underground Railroad, most of the stations were tracked down and destroyed by the Combine Civil Protection. Members of the Underground Railroad include Joe and Matt.
Possessing no means to mass manufacture weapons, the Resistance doesn't have standard issue weapons like the Combine forces do. They generally scavenge weapons from the Combine, either through killing them or finding them elsewhere. The Resistance makes use of any resources they can find, often building things out of scrapped parts and making specialized equipment, such as the Crossbow and Gravity Gun, and vehicles, such as the Scout Car, the Airboat and the Muscle Car by hand. It is however imaginable that they would have access to weapons not produced by the Combine, such as the Colt Python and Annabelle, possibly left from before the Combine were in control.