In the case of the Suez War, we need to remember that the United States had hoped between 1952-56 that Egypt under Nasser might become a reliable Western ally in the Middle East. By securing good relations with the charismatic nationalist Nasser, Eisenhower and Dulles hoped that he would influence the wider Arab League to be a reliably anti-communist bloc.

In 1954 this diplomatic engagement saw a proposed Anglo-US financed project to build a dam at Aswan, in exchange for Nasser's support for a peace plan that would resolve the Arab-Israeli conflict.

By 1956 when Egyptian forces nationalized the canal, the US state department under Dulles felt that the best solution to the dilemma would be a diplomatic negotiation between Egypt and Britain. Furthermore, Dulles and Eisenhower felt that an armed intervention to seize the canal would risk alienating Nasser and driving him further into the Soviet camp. Even worse, given Nasser's prominent role in the Arab world at the time, they feared such an armed intervention risked stoking Arab nationalist and anti-western sentiment across the Middle East.

From Eisenhower's perspective, forcing Britain (and France and Israel) to withdraw from the Suez was necessary to mitigate Arab nationalist sentiment and minimize risk of the whole region moving closer to the Soviet orbit.

The United States armed rebel groups in Portuguese Angola even though Portugal was a member of NATO at the time.

So, according to Elizabeth Schmidt in Foreign Intervention in Africa

From the outset, Angola’s three liberation movements aroused interest among the Cold War players. In the 1960s, the United States supported Portugal but hedged its bets by giving token financial and military support to the FNLA. Although American aid was not substantial enough to threaten Portugal’s hold, it did strengthen the FNLA vis-a-vis the better-educated, better-organized MPLA...

So, yes, the US did hedge bets by supporting one of the three rebel groups in Angola, the one perceived to be most American friendly at the time. And more broadly, during the Kennedy administration, there was a strong effort to build diplomatic relations with African nationalists, to keep the newly-independent countries out of the Soviet camp.

Anxious to win favor with African countries and to pressure Portugal to implement reforms that would undermine the appeal of radical nationalism, the Kennedy administration broke a longstanding tradition of support for Portugal in the UN. In March it supported an Afro-Asian Security Council resolution calling for an investigation into Portuguese repression. In June, it voted for another that “deeply deplor[ed] the large-scale killings and the severely repressive measures in Angola” and urged a speedy end to colonial rule.

But, it should also be said that the US provided a whole lot of support for the Portuguese, and managing the relationship with Portugal always was at the forefront of decision-making regarding Angola and Mozambique.

During the Kennedy years, three quarters of American military traffic to Europe and the Middle East passed through Portugal’s Azores islands air base... The administration’s Europeanists argued that American access to the base had to be protected at all costs. The administration's Africanists.....vehemently disagreed. Convinced that Salazar’s intransigence could lead to a full-scale war with Soviet involvement, Kennedy sought a middle ground. His administration quietly reduced military aid to Portugal and in 1961, over the protests of the U.S. ambassador to Lisbon, banned Portugal’s use of American weapons and military equipment in Africa. Henceforth, American military aid could be used only for NATO purposes within the area prescribed in the NATO treaty. No part of Africa fell within these boundaries.

As Salazar rejected plea after plea for self-determination in Angola, the Kennedy administration began to court the Portuguese political opposition, young middle-level army officers, and even African nationalists, hoping to turn the latter away from communism and armed struggle. Funneling covert aid through the Adoula administration in neighboring Congo, the CIA provided Holden Roberto with a $6,000 annual retainer fee, in exchange for intelligence and to strengthen the Union of Peoples of Angola/FNLA against the more radical MPLA....

The Africanist leanings of the Kennedy White House sparked an immediate backlash. In Congress, an eclectic group bearing a striking resemblance to the Katanga lobby supported the Portuguese government’s position. Composed of anticommunists and Southern segregationists, and fueled by a New York public relations firm hired by Lisbon, the congressional coalition teamed up with Europeanists in the State and Defense Departments to champion America’s access to the Azores and its special relationship with Portugal. Liberals cautioned that a pro-Portugal stance threatened to overturn policies that had successfully outmaneuvered the Soviets in Africa. In the wake of the Berlin Crisis, which lasted from June to November 1961, Europeanists gained the upper hand.

When the Azores base lease expired in December 1962, Lisbon augmented its political leverage by acceding only to temporary arrangements to be renewed at Portugal’s pleasure. American pressure on Portugal effectively ceased, and American tanks, planes, and weapons continued to be diverted for use in Portugal’s African wars....

President Lyndon Johnson, in contrast to Kennedy, took little interest in Africa....Anxious not to antagonize the former colonial powers, whose support he needed for the war effort, and determined to maintain access to the Azores air base, Johnson avoided the issue of self-determination in Portugal’s African colonies. Europeanists in the State Department became even more prominent in the shaping of U.S.-Africa policy. The shift was evident in November 1964, when American planes flew Belgian paratroopers to Stanleyville, Congo, via the Azores. Six months later, in a State Department– approved operation, the CIA arranged to sell Portugal seven B-26 bombers for use against rebels in Angola and Mozambique. While the Johnson administration provided a small amount of covert nonmilitary aid to the FNLA and FRELIMO between 1964 and 1968, by the end of his term Johnson deemed Mondlane too radical and cut off all FRELIMO support. Although American ties to the FNLA were not severed, the organization had only a weak presence in Angola.

Convinced that his predecessors had jeopardized relations with important allies, Nixon ordered a thorough review of American policies and objectives in Southern Africa, focusing on the Portuguese colonies and white-settler regimes......According to the classified study, central American objectives in the region included the protection of economic and strategic interests and the minimization of “opportunities for the USSR and Communist China . . . to gain political influence with black governments and liberation movements.” ....[T]he Nixon policy would be premised on the notion that “the whites are here to stay and the only way that constructive change can come about is through them. There is no hope for the blacks to gain the political rights they seek through violence, which will only lead to chaos and increased opportunities for the communists.” Moreover, in keeping with the study’s prescriptions, the United States would adopt an attitude of consultation, rather than confrontation, with the white regimes, maintaining “public opposition to racial repression but relax[ing] political isolation and economic restrictions on the white states.”

The new approach was readily apparent in Nixon’s public attitude toward Portugal. Rejecting Kennedy’s pressure tactics and Johnson’s benign neglect, Nixon embraced the regime of Marcello Caetano, which had replaced that of Salazar in September 1968. Viewing Portugal as a bulwark against communism and a force for stability in Southern Africa, Nixon emphasized Portugal’s role as a regional policeman.

....American foreign service officials were ordered to cease all contacts with Angolan and Mozambican nationalists, and Roberto’s retainer fee was eliminated. In August 1970, Washington officially relaxed the nine-year-old prohibition on the use of American arms and military equipment in Portuguese Africa, exempting certain dual-use items that could be used for either civilian or military purposes. Lisbon immediately bought Boeing planes to transport troops to Africa and Bell helicopters to defoliate rebellious areas. Meanwhile, American investments grew. Gulf Oil, with a huge stake in the Cabinda Enclave, became the largest single investor in the Portuguese colonies. By the early 1970s, Gulf Oil revenues provided Portugal with approximately 60 percent of its Angola war budget. The United States also bolstered Portugal’s war-weakened economy. In December 1971, Washington granted Lisbon credits worth $436 million in exchange for rights to the Azores air base through February 1974.

Emphasis mine.

I hope these passages make clear that while the US did provide some token support to FNLA and FRELIMO, that paled in comparison to the large-scale sale of planes, helicopters, chemical defoliants, and financial support that the US provided to Portugal from 1964-74. It's not at all accurate to say that the US undermined Portugal in Angola and Mozambique.

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For further reading on US, Britain and Egypt up to the Suez Crisis, check out United States, Britain and Egypt 1945-1956 by Peter Hahn.

Quoted text comes from Foreign Intervention in Africa pp 84-88

See also: The Destruction of a Nation: United States Policy towards Angola since 1945 by George Wright.