Just as a caution, don't expect anything past '95 with the 20 year rule

A disclaimer. I'm writing this while a bit pressed for time. I'll try to come back to this later. Second, I'm using the term "far-right extremism" as a convenient way to describe the movement that is statistically outside of mainstream of American political opinion, with members of that group ranging from those that limit their activity to peaceful and lawful political activism to those that flout the law or engage in violent militancy.

The roots of the far-right militant movements that sprung up in the 90s date to the Cold War and take many of their tenets and paranoid worldview from the teachings of the John Birch Society during its heydey. By the 60s there were groups like the Minutemen in California that stockpiled munitions and plotted insurgency in the case of a communist takeover, either through invasion or through betrayal by the country's own political leaders. The Terrorist Next Door, Daniel Levitas, 2002.

You mentioned the Christian Identity movement, and unfortunately I'm not too familiar with just how Christian Identity found traction in the US. However, religious extremism is intermingled with political extremism to varying extents among far-right extremists. Some far-right extremists, such as The Order, were inspired by the ideology of Christian Identity and the group's motivations were political and religious. The group aimed to overthrow the US government and incite a racial and religious war. In contrast, Timothy McVeigh appears to have flirted with white supremacy/Christian Identity but his main motivation in targeting the Murrah Building was entirely political - he aimed to punish the federal government for Waco and perhaps he envisioned it kicking off some sort of revolution (see One of Ours by Serrano). That brings me to The Turner Diaries, a horrible (both in literary style and content) little book by white supremacist William Pierce detailing a white supremacist revolution that incites World War III. This book made the circuit among those already involved in the far-right extremist world through gun shows and white supremacist publications starting in the late 70s. Among the events described in the book a small group of white supremacists initiate a campaign of terrorism in order to start a revolution, including bombing a federal building in Washington, DC. I'm not suggesting that The Turner Diaries caused the OKC bombing or the far-right terrorism in the 80s and 90s. I am suggesting books like the Turner Diaries and similar publications were indicative of a segment of the far-right extremist movement that was moving away from merely being prepared to defend against communist invasion and instead viewed things like the expansion of the welfare state and policies designed to address discrimination as signs that the communist/alien invasion had already occurred and that violent action was now justified. I think also contributing to the far-right violence of the 80s-90s was the rise of things like the sovereign citizen movement, which again moved away from the initial paranoid concern about communist subversion and towards doctrines encouraging active resistance against federal tax and firearm laws. Further adding to the rise of far-right violence in this period is the fact the abortion debate often got rolled into the existing doctrine of far-right groups. A great book on the militant pro-life movement is Wrath of Angels by Risen and Thomas. There is also the theory that hard economic times, especially in rural areas, made far-right extremism more attractive during the 80s. For more on that, check out Harvest of Rage by Joel Dyer.

To your specific question about incidents like Ruby Ridge, I think it was the natural result of having individuals ardently following a movement that mandates defying laws and having a government intent on enforcing its laws. And you see it in lesser-known incidents. Gordon Kahl, a subscriber of the sovereign citizen movement who murdered several law enforcement officers in the 80s when officers tried to arrest him when he violated conditions of his parole for a tax evasion conviction. The Covenant, Sword, and Arm of the Lord were raided by the feds in the 80s for stockpiling illegal weapons for a white supremacist revolution. I think these incidents fed into far-right extremists' existing narrative of a tyrannical government seeking to take their rights, which made "retaliatory" violence by far-right extremists more likely.

I don't have access to all of my materials on far-right militancy at the moment, but I hope this gave you a decent overview.

This pre-dates the 20 years rule, so it may be of interest and relevant to your question. One of the major events in the early 1990s that set off a lot of tensions regarding the far-right in America was the Ruby Ridge standoff. This little clip from the New York Times goes into detail about the event, but I'll add some additional analysis as to why this was such a big deal to folks in the kinds of communities you're referring to.

August 21- August 31, 1992: Ruby Ridge was a standoff between Randy Weaver, a white supremacist living in an isolated cabin in Southern Idaho with his family, and the Bureau of Alcohol, Tobacco, and Firearms. Weaver had an outstanding warrant for failure to appear in court, but as an active participant in the pro-gun and white supremacy communities, was a person of interest to the ATF. An undercover ATF agent met Weaver at a meeting for the Aryan Nations, the first time Weaver had attended such an event. The agent and Weaver had additional contact over the next 3 years. According to the ATF, Weaver sold two illegal sawed off shotguns to the ATF agent, with this more biased video clip claiming they were less than half an inch too short.

The ATF then took an increased interest in Weaver, surrounding his cabin with armed agents. Weaver had his wife, daughter, son, a newborn child, and a family friend with him in the cabin. The key to this story was that the ATF agents were given a different rules of engagement that permitted them to fire at any of the adult men on sight. After Weaver's dogs alerted to the presence of someone else near the cabin, Weaver, his 14 year old son, and the family friend went to investigate. They encountered a group of agents that they had not known about, at a Y in the trail. There was an exchange of fire with accounts differing to who shot first, but Weaver's son was gunned down and an ATF agent was killed.

The ATF continued to have the shoot-on-sight rule of engagement with the people in the cabin. After the siege carried on for a bit, Weaver exited his cabin to try and retrieve his son's body*. An ATF agent fired at him, causing him to run back towards the cabin. That agent, Lon Hourichi, fired several more shots at Weaver as he ran to the cabin door. One of those shots penetrated the door, striking Vicki Weaver (wife) and killing her, allegedly while she held their infant child.

The siege concluded peacefully a short time later, and the Weaver's ultimately won a lawsuit against the federal government, because of the rules of engagement that were deemed to violate Weaver's constitutional rights.

Now, here's where this gets interesting (and slips in just before the 20 year rule). Less than a year later (between February 28 and April 19, 1993), the Waco Siege took place, where the Branch Davidian compound in Texas caught fire after an initial firefight and standoff. On that video link, listen to what happens at 1:30. The live news coverage puts an imprisoned member of the Branch Davidians on the phone. He gives a vitriolic condemnation of what the media had been doing to David Koresh, the BD leader, calling it character assassination and propaganda.

There were some similarities between the two incidents that are worth noting. First, in both sieges, the ATF was the one doing the besieging. Second, Lon Hourichi, the ATF agent who killed Vicki Weaver, was also at the Waco Siege. Both of the incidents involved pro-gun far-right groups being held in their own homes by the federal government (which, let's not forget, was in the first two years of the Democratic Clinton administration).

The Branch Davidian compound burned to the ground on April 19, 1993.

Timothy McVeigh detonated his bomb at the Oklahoma City Federal Building on April 19, 1995, the 2-year anniversary of the events in Waco.

It was, in McVeigh's own words, retaliation for the previous two events.

*Edit: as /u/Rittermeister corrected me, the Weaver's had previously recovered Sammy's (the son) body. As per Wikipedia:

Before the negotiators arrived at the cabin, FBI sniper Lon Horiuchi, from a position over 200 yards north and above the Weaver cabin, shot and wounded Randy Weaver in the back with the bullet exiting his right armpit, while he was lifting the latch on the shed to visit the body of his dead son.

Hi there /u/SapphoInDecay! I actually was a teaching assistant for a class back in the early 2000s, during my graduate school years, called "Conspiracies and Conspiracy Theories in US History and Politics," which focused on the roots of consipracist thinking in the US. It may be of interest to you if I suggest some readings from that class, since most of the responses you currently have are bumping up pretty heavily against the 20-year rule.

A quick and readable essay is Richard Hofstadter's The Paranoid Style in American Politics, published in Harper's magazine in 1964. Hofstadter's essay is a good overview of various (feared) conspiracies in US history, from the worry over the Illuminati through the anti-Masonic movement, the Know-Nothing party, anti-Catholicism, anti-Mormonism and more. One of the "selling points" of Hofstadter's essay -- in other words, why I'd still use it in class -- is that it's really good at drawing parallels between those types of conspiracy theories and ones at a later date, anti-Communism being the parallel in the 1960s.

The prof who was my advisor still maintains his class blog, and has some readings on it that are more accessible than the official published versions (although any of the official versions are easy to find if you have access to JSTOR, or access to a university library). This is a great starter/roundup, which I won't discuss here to avoid the 20-year rule, that will illustrate ways in which some of the militia movement ideas still resonate today.

Check out the book For the Kingdom and the Power. It is about the marketing power and how the KKK came to fruition via pr and such. Fascinating read

http://www.lakeclaremont.com/#!for-the-kingdom-and-the-power/chc0

I don't have much analysis for you, but you can find an archive of KKK material in the 1920s from Washington State here(U of Washington material):

http://depts.washington.edu/civilr/kkk_intro.htm

To give it my gloss, its very anti Catholic, anti Communist, and very nativist. I would infer that those ideas descended into the far right of the time period you describe (although I don't have citations or primary sources to back that up- just the similarity in thought).