Hi OP, I'm currently working on my undergraduate thesis, which is, essentially, a study revolving around the 1973-1976 period in Argentina and the role of paramilitary agencies such as the Triple A (Alianza Anticomunista Argentina) and their counterproductive role in the survival of Isabel Perón's government. Due to this, I can only really discuss the Argentine case in-depth, yet I can anecdotical mention that in the case of the 1968 Peruvian coup, it was clear for both the State Department and the CIA that a plot to oust president Fernando Belaúnde Terry (arguably a centre-right reformist) was in the making, yet they failed to diagnose that the coup would give way to "revolutionary" military rule for a 12 year period (Pease & Romero, 2013). That said, they do not seem to have played an active role in the coup.

Going back to the Argentine case, however, we come across a similarly passive stance from American authorities. When taking into account the March 1976 coup against Isabel Perón, however, we need to do away with any parallels which may come to mind with the downfall of Allende in Chile just under 3 years before. In the Chilean case, Allende was "problematic" to both the Chilean military and American interests in the region (Richard, 2017). When looking at Isabel Perón, however, the United States never formulated a strong stance as to whether she was a handicap to their regional interests or not (Yofre, 2006). In of itself, the military only decided that they were aiming to remove her several months into her tenure (we're talking about late 1974 to early 1975). Only by March 1975 did they decide that she was "unfit to pacify the country" (Reato, 2020).

As you've pointed out, the subsequent military dictatorship, the "Proceso de Reorganización Nacional" or National Reorganisation Process did enjoy of American support, yet never in the same overt degree as that enjoyed by the Brazilian 1964-1985 military junta (Reato, 2020). The reasons behind this state of affairs are, I believe, varied to say the least. Let's first assess the nature of the regimes which had been overthrown in both Chile and Brazil. When discussing the former, we're addressing the first democratically elected left-wing political in the region, Salvador Allende, whose platform posed, per the State Department and a number of intelligence reports, "an obstacle to American interests" (Collier, 1996). The latter example, Brazil, displays a number of similarities to the Chilean case. After all, intelligence reports label Joao Goulart as a reformist "seeking to minimise the influence of the private sector" in Brazilian public life (Braga et al, 2004). To the United States, these political agents represented either geopolitical or strategic, resource-driven setbacks to American interests in the region.

The story with Isabel Perón is, however, considerably different. While Juan Domingo Perón had been elected on something of a left-leaning platform in 1973, being the candidate of the "Frente Justicialista de Liberación" or FREJULI (Reato, 2020), soon after forming his government, he severed his ties with the left-leaning factions of the Peronist movement, publicly ostracising the "Montoneros" in a famous speech delivered from the balcony of the "Casa Rosada" (Larraquy, 2019), leading to the "Montoneros" once again employing various forms of urban terrorist strategies as a means to attain their goal of the "patria socialista" (Larraquy, 2019). This, together with the unwavering attacks launched by the "Ejercito Revolucionario del Pueblo" would lead Perón, his wife Isabel, and their advisor José López Rega to establish the aforementioned Triple A.

With Perón's death in 1974 and Isabel's rise to power as president (let's keep in mind she had been elected as vice-president in the FREJULI's ticket), the influence enjoyed by López Rega and the right-leaning elements of the Peronist movement heightened (Servetto, 2007). However, thanks to López Rega's toxic hold on Isabel and his personality 'quirks' (Larraquy, 2019), the counter-insurgence strategy adopted by Isabel Perón's government only made matters worse, further antagonising the "Montoneros" (who had started as the main representatives of the Peronist left). This, together with mediocre economic management brought about by Celestino Rodrigo (Aznárez, 2007), just made of life in Argentina even more of a hectic prospect. These factors would in time just convince almost the entirety of the military establishment (Yofre, 2006) and the opposition (Reato, 2020) that a coup was not only inevitable, but "necessary". The United States was fully aware of the plotting in the barracks since at least a full year before it came through, as detailed by a wide array of declassified documents made public in the last few decades (I'm happy to share these with you in case you're interested). However, their support for Videla and his successors only became a material reality until after the March 1976 coup.

Why? Well, if you were to ask me, it would probably relate to a number of things. The first of these has already been addressed throughout the length of my post - the United States did not perceive Isabel Perón and her administration as an obstacle to American regional interests. In addition to this, the American posture towards Peronism following the mid-1960's is characterised by a high degree of apathy (Diamond, 1998). The State Department and the CIA, together with several Argentine military entities attempted to neutralise Juan Domingo Perón during his exile, yet for the most part failed to destroy his network - see how he directly influenced the election of Arturo Frondizi in 1958 (Larraquy, 2019). Finally, and here I'm sinning by speculating, I believe there may have been a chance that to American authorities, Isabel's government may have been able to do away with most of the "failings" which had motivated the military into plotting against her. After all, her administration had launched a bloody, albeit disorganised counter-insurgence campaign agains the "Montoneros" and the "Ejercito Revolucionario del Pueblo", and the economic misfortunes of Argentina had become commonplace to international observers since the early 1950's (Lewis, 1990).

In short, most cases of American intervention were either deemed preemptive or corrective by the perpetrators, and in the Argentine case, the nature of said interventionism broke the model, becoming instead a case of geopolitical opportunism.

The United States has always been ubiquitous in the modern history of the Western Hemisphere. I’m a specialist in Argentina’s history (specifically its twentieth-century, Cold War-era history), and like you said, it is well-known that the United States not only supported Argentina’s Proceso de Reorganización Nacional dictatorship (1976-1983), but any anticommunist government in the region at the time. You also mentioned Operation Condor. To clarify, Operation Condor was a US-backed information exchange program. However, it also was instrumental in greasing the gears of a repressive machine guilty of the abduction, torture, murder, and/or disappearance of tens of thousands. While the individual states that ultimately carried out this repression had the ability to do so using their own modes of repression independently from each other, it was what Menjívar and Rodríguez (2005) call the “U.S.-Latin American interstate regime” that accelerated and amplified this process.

The Eisenhower administration can be considered the origin of modern US support of repressive states in Latin America. In a 1954 report compiled for President Eisenhower, General James Doolittle wrote that the United States is “facing an implacable enemy whose avowed objective is world domination by whatever means and at whatever cost.” Therefore, “long-standing American concepts of ‘fair play’ must be reconsidered.” Doolittle expressed the need for the United States to find ways to “destroy our enemies by more clever, more sophisticated, and more effective methods than those used against us.” It was a few years later in 1959 that General Doolittle’s advice was heeded. The Cuban Revolution served as a turning point in that military and foreign policy experts in the US government realized that it needed to drastically redesign its tactics and preexisting political apparatuses to combat communist-inspired subversion, both real and alleged. The main tool through which US political interests expressed their will over Latin America was the US Army School of the Americas (SOA). This military training center, located in the Panama Canal Zone, was where military officers from across the region were trained in antisubversive tactics, including torture. Many of those intimately involved in the Cold War-era dictatorships of Latin America received some training at the SOA, including General Leopoldo Galtieri of Argentina, General Hugo Banzer Suárez of Bolivia, General Augusto Pinochet of Chile, Major Roberto D’Aubuisson of El Salvador, General Manuel Antonio Callejas y Callejas of Guatemala, General Policarpo Paz García of Honduras, and General Manuel Antonio Noriega of Panama. Other officers that received counterinsurgency training at the SOA included nineteen of the twenty-seven army officers in El Salvador who murdered six Jesuit priests in November 1989, as well as four of the five officers accused of organizing the Honduran Battalion 3-16. Because of the American origins of the training many of the perpetrators of Operation Condor received, any of the various militias, death squads, and task forces that operated across Latin America, government-sponsored or otherwise, bore the structure of a US Special Forces unit.

Operation Condor was also conducted out of the Panama Canal Zone, specifically out of Fort Gulick. It was an information exchange apparatus through which dictatorial governments kept in contact. This was mainly done through Condortel, a series of computerized intelligence databases and telecommunications systems that, as an Argentinian military source told a U.S. embassy contact, the CIA had been integral in establishing given that no country in Latin America had the technological capacity to set up such systems. This claim was confirmed by a declassified cable sent by the ambassador to Paraguay Robert White to the Secretary of State Cyrus Vance in 1978. The main aim of the Condor system was to aid in the destruction of subversive threats to these governments. It also aided in military coordination, allowing these states to permit a neighboring country’s troops within their borders for the purpose of locating and eliminating targets. This was how even those exiled from their country were targeted and assassinated, including former Chilean Army chief General Carlos Prats, former president of Bolivia Juan José Torres, and Paraguyan opposition leader Agustín Goiburú.

Probably the most brazen act of political repression carried out by any of the countries participating in the Condor system was the assassination of Orlando Letelier, a Chilean diplomat and a member of the cabinet of the socialist Chilean president Salvador Allende. Allende had been overthrown by a US-backed military coup led by Augusto Pinochet in 1973. In 1976, Letelier was living in exile in Washington DC when his car exploded while rounding Sheridan Circle. It is now known that the car bomb that killed Letelier was planted by agents of Chile’s National Intelligence Directorate (DINA). While there is ongoing debate whether American diplomats had prior knowledge of Letelier’s murder, it has been shown in documents from the State Department that Henry Kissinger, in his capacity as Secretary of State, refused to warn Pinochet’s government against carrying out assassinations outside the borders of Chile.

Chile has been consistently pointed out as one of the cases where the United States was most intimately involved in the overthrow of a democratically-elected government in Latin America. However, it was not only through military measures that the United States exerted their influence. Immediately upon the election of Salvador Allende to the presidency, the Nixon administration placed enormous economic sanctions on the country in the hope of either preventing Allende’s reelection or his overthrow by civilian opposition leaders. Nixon actively sought to instigate a coup in Chile immediately upon Allende taking office. The Director of the CIA Richard Helms took some basic handwritten notes during a September 15, 1970 meeting with Nixon and Kissinger: “$10,000,000 available, more if necessary. Full-time job — best men we have. Game plan. Make the economy scream.” It is not surprising that the United States took such an active role in the overthrow of Allende, since Chile at the time was arguably the most stable democracy in the region. This mainly had to do with the nature of the Chilean military, with the leadership maintaining strict political neutrality under army commanders René Schneider and Carlos Prats. It was only after the anti-Allende opposition forced Prats to resign that Augusto Pinochet, a man less opposed to the autonomy and politicization of the military, was made Chile's army commander.

The other major case study of US involvement in Latin America is Guatemala. In the same year that General Doolittle compiled the report quoted above, the Eisenhower administration infamously aided in the overthrow of the democratically-elected president Jacobo Arbenz. Arbenz was demonstrably not a communist or even a socialist, but rather sought land reform in his country, seeking to redistribute land owned and unused by large agricultural corporations. The United Fruit Company, an American corporation trading primarily in Central American bananas, was one of the largest landholders in the country affected by such land reform. One of the main historical narratives regarding the 1954 coup d’état in Guatemala is that the United Fruit Company was a particularly powerful corporation, maintaining connections with many of the top US government figures of the time: Secretary of State John Foster Dulles had worked as one of the company’s lawyers, while his brother and CIA Director Allen Dulles served on the company’s board of directors. Eisenhower’s ambassador to the UN Henry Cabot Lodge was a major shareholder in the company, and Ed Whitman, one of United Fruit’s public relations figures, was the husband of Eisenhower’s personal secretary. With the political developments under Arbenz, it has been speculated that Guatemala would have become a Cold War battleground regardless of the involvement of United Fruit. However, it cannot be denied that the actions taken by figures associated with the company expedited the process of US involvement in the country. Arbenz was overthrown in a coup led by Carlos Castillo Armas, instituting an anticommunist dictatorship and creating the environment necessary to spark a 36-year-long civil war in the country that finally ended in 1996. US support for anticommunist regimes in Guatemala, Nicaragua, Honduras, and El Salvador, which were not only friendly towards US political interests but commercial interests as well, led to decades of political instability and the deaths of thousands.

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See the answers by /u/ainrialai and /u/iconicjester in Why did the US overthrow so many democratically elected governments in Latin America? And why did they replace those governments with fascist dictatorships instead of liberal democracies?

EDIT: /u/aquatermain contributed to the answers at the link, among others.