One of the main actors that assisted in the creation of the FCE was, ironically, Japan. Their intention was certainly not to solidify a new, unsure nation a week before the referendum on that nation was to be voted on by all of its citizens. This was what occurred with the Japanese attack on Pearl Harbor and subsequent declaration of war against the United Nations, the Netherlands and the United States of America on November 9th, 1942.
The referendum on the constitution of the Federation of Central European States was scheduled for November 15th, 1942. At that point, Czechoslovakian military units were already advancing into the Japanese puppet of Menkukuo, a drive that would eventually cause that nation to surrender unconditionally shortly after the new year.
Those frontline units included not only Czechs and Slovaks, but also Austrian and Hungarian volunteers who had joined the Czechoslovakian army to fight the communists or to advance the cause of a central European union. The stories of Czechs, Slovaks, Hungarians and Austrians fighting together, as well as the now shared threat (though distant) of the Japanese caused a sense of national identity and pride which many consider to be the crucial factor in the success of the National Referendum in all four states of the Federation.
This good news came with it the introduction of several new countries into the United Nations. The Netherlands joined as soon as practical, on November 11th.
The United States, still a bit concerned at joining an alliance that comprised most of the Old World, delayed becoming a member until November 20th. In the end, they simply could not refuse to coordinate with the battle tested democracies that comprised the United Nations and thus prolong the war. It is of note that the Philippines and Liberia also joined the United Nations, though allowing the United States to represent them in all decisions.
An odd footnote in this global realignment is Columbia, which declared war on Japan shortly after Pearl Harbor, but refused to join the United Nations. It hardly mattered in the end, as Columbia signed a Peace Treaty with Japan within two months that normalized all relations. No casualties were recorded on either side of this paper war.