The US isolationist foreign policy really shifted with Pearl Harbor and the US entry into World War II, before the Truman Doctrine became the primary motivation. Keep in mind that the isolationist policies of the US related to European affairs. As far as the America's went, the US had a strong interventionist policy already (i.e., the Monroe Doctrine). While the US did get involved in WWI, after the war was over, it quickly retreated back to its isolationist tendencies and refused to join the League of Nations. With World War II, however, the US again became involved in European affairs, but not until the attacks on Pearl Harbor. Unlike the end of World War I, however, the US under Roosevelt backed the establishment of the UN, supported the Marshall Plan to rebuild Germany/Western Europe and Japan, and soon the US was competing against the Soviet Union for influence around the globe during the the Cold War. Whether something is a good thing is terribly subjective. Some things are good about interventionist policies, and some are bad. It certainly would have been cheaper to remain isolationist. However, we have not had World War III, which most would agree is a good thing. Further, US influence politically and economically around the world is enormous. During the Cold War, No US President officially disagreed with the idea that the US should not retreat into isolationism. The Cold War was a continuation of the WWII idea that we were heroes fighting sinister foes.

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Part of the problem in identifying when the United States stopped being isolationist is that the term when applied to American foreign policy as a whole pre-WWII is a misnomer. The traditional story says that the United States burst onto the international scene during WWI, but faded away when Woodrow Wilson failed to convince the United States Congress to ratify the Treaty of Versailles and thus to bring the United States into the League of Nations that he himself had envisioned. In reality, however, the United States remained more of an independent internationalist nation than an isolationist one during the 1920s. The United States negotiated the Washington Naval Treaty in the early 1920s, an anti-war treaty that limited the number of warships that Japan, Britain, the United States, France, and Italy could possess at a time. Not only was this treaty a proactive step towards preserving international peace, but it was also strongly supported by some of the same Senators, notably Senator William E. Borah, who had opposed the ratification of the Treaty of Versailles. This was not the only example of American involvement in European affairs. Indeed, the Commerce and State Departments remained very active in Europe, securing American trade on the continent. And, most famously, Americans led efforts to resolve an international crisis that erupted between France and Germany over the payment of reparations, culminating in the Dawes and Young Plans of 1924 and 1929.

After the Great Depression and the rise of Fascism in Europe, however, the American Congress tried to draw the United States out of European affairs with the Neutrality Acts, which barred the United States from trading with belligerent nations. Many in Congress rightly concluded that American trade with Britain and France had helped to draw the United States into WWI, which Americans generally regarded as a mistake at the time (there were even accusation that "Merchants of Death" had tricked Wilson into war, accusations that the Senate investigated and found to be untrue). But while the American public stood behind Congress in this regard, president Franklin Roosevelt wanted instead to assist France and Britain in their potential struggle with Germany. In fact, during the Sudeten Crisis, Roosevelt offered at the last minute to act as an intermediary in order to ensure that Britain and Germany would not again go to war, and he only withdrew his offer after receiving a guarantee from Neville Chamberlain that the British Prime Minister would appease Hitler at the (now infamous) Munich Conference. After Hitler made a fool of the Western leaders by occupying the rest of Czechoslovakia, Roosevelt then moved to have a loophole in the Neutrality Acts extended so that he could supply arms to Britain if he needed to, a request that Congress rejected. Thus as war approached in Europe, the United States found itself with headed by a strongly isolationist Congress and an equally strong advocate for the support of Britain's efforts with Congress's ideology winning the day. This, however, was hardly an era of pure isolation, as popular memory suggests. Indeed, once war actually began between the European powers (and here I mean the German invasion and occupation of France in 1940 and not the Franco-British declaration of war issued against Germany i 1939), Roosevelt convinced Congress to provide him with a loophole to the Neutrality Acts, the Lend-Lease Act which allowed him to "lend" war material to Britain in exchange for the "leasing" of British bases to the United States. For all the isolationist tendencies promoted by Congress and the American population, FDR still found a way to aid Britain in its time of need.

On top of all of this, FDR simultaneously moved to counter Japanese aggression in China -- with war raging in Europe, in July of 1941, FDR placed an oil embargo on Japan in response to the extensive imperial intervention of Japan in China. So while he was now allowed to supply the British war effort, FDR also moved to use American economic might to stale the aggressive expansion of Japan, a German ally. It's quite odd that most Americans remember the United States as being in total "isolation" before the attack on Pearl Harbor when you consider just how involved the United States was in the war economically if not militarily prior to the attack.

Once Japan attacked Pearl Harbor and Hitler subsequently declared war on the United States, FDR had the full power to implement the interventionist foreign policy that he preferred. This meant that the United States employed a "Europe first" policy despite facing Japan in the Pacific, and that the American and British war effort would become intimately intertwined (again, as FDR preferred). Once the Soviet Union joined the war effort, it also meant that FDR recognized an opportunity to create a new world system that would guarantee that the United States remained active in world affairs even after the war ended. FDR oversaw the creation of the "Bretton Woods" system of international finance (named after the American city in which the system was negotiated), which linked all currency exchange rates to the dollar and created the IMF and the World Bank in the process. He also led the effort to create a permanent "United Nations" to replace the League of Nations, and he worked hard to ensure that Stalin would remain a part of this alliance even after the defeat of Hitler. FDR benefited here from a change in the American perception of the world. Policymakers and the population alike both came to see America's failure to involve itself during the Sudeten Crisis as a mistake that ultimately led to a war that drew the United States in anyway. After Pearl Harbor, Americans perceived that they had no choice but to be involved in world affairs. When FDR died in 1945, he had already laid a foundation that ensured American involvement in world affairs after the war.

Nevertheless, when the war did end, many in Congress wanted the United States to step down from its leading role that it had adopted during the war. This, in part, stemmed from the fact that most people were not that afraid of the Soviet Union. But Truman, influenced in part by the diplomat George Keenan (who argued in his famous secret "Long Telegram" and public "Article X" that the Soviet Union was not a normal nation but one bent on world domination and that its ambitions needed to be "contained"), viewed Soviet action for more suspiciously and called for measures to counter Soviet ambitions. After the Communist "coup" in Czechoslovakia, however, many in Congress came to fear the Soviet Union and supported Truman's efforts at containment, including funding anti-communists in Greece and Turkey, the implementation of the Marshall Plan, and the creation of NATO. Thus while Truman inherited an American foreign policy intricately linked to global affairs, he also presided over a transitional period in American politics in which the United States, from its leading position, took unilateral actions to counter Soviet interests since the United Nations failed to live up to FDR's expectations after the dissolution of the Soviet-American alliance that had defeated Hitler.

Did the new interventionist policy provoke conflict with the soviet union or was it inevitable?

Historians these days tend to shy away from arguing that the Cold War was "inevitable," but distrust between the United States and the Soviet Union emerged even while the two continued to battle Hitler's Germany together. Given that the two nations both perceived real interests in the evolution of European politics, the foundations for the Cold War most definitely existed prior to any single action made by Truman or Stalin.

Was the change in foreign policy an overall good thing?

This is an extremely difficult question to answer, although I think it's fair to say that American involvement with Western Europe (which it rebuilt with the Marshall Plan and then protected through NATO and the threat of nuclear retaliation against the Soviet Union) was more fruitful than its involvement in the so-called "periphery" or "third world" where American policymakers continuously mistook an alliance with the Soviet Union for proof of a monolithic communist threat. The premier example of the latter is of course the Vietnam War, where the United States wasted lives and dollars only to withdraw anyway without a monolithic communist government forming in the region (in fact the communist nations of Cambodia and Vietnam went to war with each other in the years after American withdrawal). Vietnam is not the only example, however. Even Truman, the original architect of America's successful European policy, made a similar mistake by involving America in the Korean war. If you have a more specific question, I will try to answer it.

Did different US Presidents have completely different opinions about the subject?

One might make an arbitrary distinction between, say, LBJ (who escalated the war in Vietnam) and Nixon (who initiated the period of "Detente" with the Soviet Union), but both of these presidents actually sought the same things: a honorable end to the war in Vietnam and arms reduction talks with the Soviets. But of course every president approached the Cold War differently. Reagan, for example, called for an end to Detente with the Soviet Union. If you have questions about specific presidents, it will be much easier to answer you since certain consensuses did exist, most notably that the American way of life trumped anything the Soviets could offer and would defeat Soviet goals in the long run, while each president approached this battle differently, with some presidents (LBJ and Nixon again) perceiving a Soviet-American thaw as the most beneficial path forward for the United States.

And, finally: