Alliance among Superpowers (1945-50) Made for World Peace
A short-term peace in the world when the superpowers were aligned with each other
Superpowers were aligned
I understand "secure" to mean lack of armed conflict between nations. To answer this question let us take a look into the world when there were three superpowers. That was a time when the superpowers were aligned with each other: August 1945 to June 1950. During this period the Axis powers (Hitler's Germany, Italy, Japan) had been defeated by the Allied forces led by the United States, Great Britain and Russia. These three are referred to as the Big Three. At the Yalta conference in February 1945 they decided the fate of the world. Italy had been subdued by the Allied forces under the command of Field Marshall Bernard Montgomery of Great Britain and Major General Dwight Eisenhower of the United States. Benito Mussolini, former Fascist dictator of Italy had been ousted by the Italian Council, incarcerated who escaped only to be captured and hanged in 1945. Germany was at the verge of defeat. It was only a matter of time that Japan would be defeated. The Allied forces to subdue Japan was being prepared, assembled in Australia by Gen. Douglas MacArthur, commander of the Southwest Pacific theater, and Admiral Chester Nimitz. (The command of the Pacific was split between them to give a chance for the Navy to redeem itself having been the first casualty of the Pacific war at Pearl Harbor.) The Allied grand strategy was "Europe First" meaning Italy and Hitler's Germany was to be defeated ahead of Japan. In June 1945 Germany surrendered unconditionally. That means no more conference to stipulate the terms of surrender of Germany as what happened in WWI. Germany was to be ruled and occupied by four powers: United States, Great Britain, Russia and France.
France was not allowed to participate in the Yalta conference because it was taken for granted that it would follow the lead of Great Britain, although France was already liberated in September 1944 yet. Nationalist China, under Generalissimo Chiang Kai-shek had a sizable army but was not allowed to participate in the same conference because it was assumed to follow the lead of the United States. Besides, the leadership was being contested in a civil war by Mao Zedong of the Communist Party and Chiang who was supported by the United States.
The civil war that ended in 1949 is not considered a disturbance of world peace. Even before WWII the communists and the nationalists were at war. When the Japanese wanted to dominate China, Mao and Chiang agreed to form a tactical alliance to drive away the Japanese and when this job was done would resume their civil war. All the while the US was supporting Chiang with warplanes, guns, and 153,000 military personnel including 53,000 marines and topnotch generals like Gen. Joseph Stilwell, Major Gen. Albert Weldemeyer, and Gen. George C. Marshall. Mao, however, did not want to declare war on the US because that would turn the civil war into international war at the disadvantage of Mao's forces. The US did not declare war on Mao's forces either.
In August 1945 Japan surrendered. The United States occupied Japan under the command of Gen. MacArthur who was the virtual emperor. The reason was that the Japanese emperor was under his command. One of the terms of surrender of Japan was that the emperor shall not be abolished. MacArthur himself advocated this because it was easier to control Japan with the emperor. A larger armed force would be needed to rule Japan without the emperor; MacArthur needed only 250,000 to control Japan (Hunter, F. The Untold Story of General Dougjas MacArthur. 1954).
So, the world was at peace since August 1945 to 1950. Why so short a time?
Even before WWII there was already an incipient rivalry between Russia and the west. Russia wanted to play safe with Germany that is why Joseph Stalin, premier of Russia, entered into a secret non-aggression pact with Hitler. Stalin did not prepare for a war with Hitler who gobbled up Europe. Italy moved up to Africa, conquered Ethiopia, bypassed Spain that stayed neutral (it was under the dictatorship of Generalissimo Franco anyway). Great Britain was the only European power that stood in the way of Germany. At that time, before 1941 (Japanese attack on Pearl Harbor), the United States still isolated itself from the war.
Stalin was taken by surprise when Hitler attacked Russia. Early on Russia was on the defensive, the Russian winter staved off its defeat. Stalin called for help from the west to mount a second front for Hitler. The United States was tooling for the counter offensive against Hitler. At the Yalta conference Stalin grumbled that the west stalled its help for Russia, or rather counter offensive to Hitler, by design to weaken Russia. May be one of the means to mollify Stalin was to allow him to have three votes in the United Nations that was yet to be organized. The United States and all others in its orbit was given one vote each country, including Great Britain and France. One reason why Pres, Franklin D. Roosevelt of the US and Winston Churchill, prime minister of Great Britain, deferred to Stalin could be that Stalin's lead was being followed by 11 countries in eastern Europe. At Yalta Roosevelt, at the advice of Gen. George C. Marshall, chief of staff, urged Stalin to join in in the defeat of Japan. Stalin agreed who dispatched an army to Korea to drive away the Japanese there. The US alone bore the brunt to land forces in Japan. Great Britain mopped up Japanese forces in the far east under the leadership of Lord Mountbatten.
The incipient rivalry between the US and Russia showed up in Korea. The Russian expeditionary force landed in Korea ahead of the American forces. Russian forces were about to go past Seoul. Gen. MacArthur, whose command included Korea having been made sole commander of the Southwest Pacific theater, had to rush to get the approval by the US and Russia that a boundary be established for their respective operations. The 38th parallel was agreed upon as the boundary, that has stood up until today.
The United States operated on the south of 38th parallel, Russia operated north of it. They had opposing policies toward Korea. South Korea and North Korea conducted separate elections to install their own government claiming jurisdiction over the whole Korean peninsula. Then in June 1950 North Korea invaded South Korea.
End of the peaceful interlude. Enter the Cold War. Since then there has been no peace in the world.
Room for disagreement
There is room to disagree that there was a time when the United States was the sole superpower. By sole superpower I understand to mean no other country could contest the United States in terms of dominance. Dominance is in terms of power that is divided into naked (force), persuasive, priestly, economic and executive. This is the subject of a Hub I am preparing.