I'm not sure why people downvoted this question. It's a good one, but a little tricky to answer.

I'm going to take your questions one by one, if that's alright.

Is it really as simple as keeping a new country from the USSR?

No. More on that later!

Was it just by that point obvious that there would be a Jewish state in Mandatory Palestine, and the US wanted it for itself?

Partially, more on that also later!

Did the US not recognize how incredibly harmful the decision would be to relations with Arab nations?

It guessed to some extent.

How did we get from (1) the dire warnings about consequences of creating a Jewish state in the area described in the King-Crane report to (2) "might as well"? What US government actions over that time period contributed to or detracted from the Zionist cause?

There are a lot of them, fortunately, for us to examine!

I don't get how it was politically seen in the US as the right thing to do, and I think I must be missing some US involvement or perspective on Zionism pre-1946.

Keep in mind that it's not always "politically" right, but also "moral" that may have had a small effect. Still, I'll expand on all this below.

Now, let's go ahead and expand on all of these things.

When the US government made the decision to support the creation of Israel, there are two stages of that. The first is the decision to support the partition plan of 1947, and the second is to provide de facto recognition of Israel after its declaration of independence on May 14, 1948 (taking effect May 15, 1948).

The United States supported the partition plan of 1947. How it came to support the partition plan is another story entirely. Let's start with the King-Crane Report. This report, an investigation in 1919 created by the United States, judged that the creation of any state in the Middle East was impossible and dangerous. It believed that the people were not ready yet, and that Mandates should be established. You can view the report here, and see what it says:

E. We recommend, in the fifth place, serious modification of the extreme Zionist program for Palestine of unlimited immigration of Jews, looking finally to making Palestine distinctly a Jewish State.

For "a national home for the Jewish people" is not equivalent to making Palestine into a Jewish State; nor can the erection of such a Jewish State be accomplished without the gravest trespass upon the "civil and religious rights of existing non-Jewish communities in Palestine." The fact came out repeatedly in the Commission's conference with Jewish representatives, that the Zionists looked forward to a practically complete dispossession of the present non-Jewish inhabitants of Palestine, by various forms of purchase.

In view of all these considerations, and with a deep sense of sympathy for the Jewish cause, the Commissioners feel bound to recommend that only a greatly reduced Zionist program be attempted by the Peace Conference, and even that, only very gradually initiated. This would have to mean that Jewish immigration should be definitely limited, and that the project for making Palestine distinctly a Jewish commonwealth should be given up.

There would then be no reason why Palestine could not be included in a united Syrian State, just as other portions of the country, the holy places being cared for by an International and Inter-religious Commission, somewhat as at present under the oversight and approval of the Mandatary and of the League of Nations. The Jews, of course, would have representation upon this Commission.

The recommendations now made lead naturally to the necessity of recommending what power shall undertake the single Mandate for all Syria.

The King-Crane Report thus called for the world to establish a Mandate in Syria, which would eventually lead to sovereignty for Arabs and Arabs alone in the region. This was done just after the fall of the Ottoman Empire, and Syria would be joined to Palestine. Necessarily, things changed as the situation did. Emir Faisal, recommended to lead Syria and include Palestine in this state under Mandate, was removed from power by the French in 1920 after the British stopped trying to protect him. He was then sent to lead Iraq, while his brother stayed in power in Jordan if memory serves. Palestine never joined Syria. The Zionist program was not curtailed strictly, nor did the United States anticipate what was to come: the riots in 1920, 1921, and 1929, the revolt in 1936, the Holocaust and the displaced Jews who remained scattered throughout Europe in the hundreds of thousands, and more. All of this, as you may imagine, changed significantly the United States' position. I expand more on the events themselves in a question in the FAQ, this one. But just to give you an idea of the scale of difference, Jews were numbered at 83,000 in the 1922 British Census, while the population was 757,000 in full. Jews were barely more than 1/10 of the population. By 1947, the Jewish population had become 1/3 of the population. Things had changed. Refugees from Europe were trying to get into Palestine: after Israel was established, they and Jews from Arab countries (some expelled, some leaving of fear, some of their own free will) doubled the population of Jews to outnumber the initial Muslim population that had been there before the war.

Things were different, and so US policy changed. The US gradually became more sympathetic to the Zionist cause, especially after the Holocaust. The Anglo-American Commission, for example, wrote recommendations in 1946 that suggested that:

Recommendation No. 1. We have to report that such information as we received about countries other than Palestine gave no hope of substantial assistance in finding homes for Jews wishing or impelled to leave Europe.

But Palestine alone cannot meet the emigration needs of the Jewish victims of Nazi and Fascist persecution; the whole world shares responsibility for them and indeed for the resettlement of all "displaced persons".

Obviously, this was a big point. Further:

Recommendation No. 2. We recommend (a) that 100,000 certificates be authorized immediately for the admission into Palestine of Jews who have been the victims of Nazi and Fascist persecution; (b) that these certificates be awarded as far as possible in 1946 and that actual immigration be pushed forward as rapidly as conditions will permit.

Also:

Recommendation No. 3. In order to dispose, once and for all, of the exclusive claims of Jews and Arabs to Palestine, we regard it as essential that a clear statement of the following principles should be made:

I. That Jew shall not dominate Arab and Arab shall not dominate Jew in Palestine. II. That Palestine shall be neither a Jewish state nor an Arab state. III. That the form of government ultimately to be established, shall, under international guarantees, fully protect and preserve the interests in the Holy Land of Christendom and of the Moslem and Jewish faiths.

Thus Palestine must ultimately become a state which guards the rights and interests of Moslems, Jews and Christians alike; and accords to the inhabitants, as a whole, the fullest measure of self-government, consistent with the three paramount principles set forth above.

This, the one-state solution, was the favored proposal of both the United States and the Soviet Union. When Gromyko made his speech to the UN before the UN Special Committee on Palestine (UNSCOP) made its report, he said a one-state solution was best, but that if this was unworkable that partition would be better. This idea had slowly gained on people since the British proposed it in 1937 with the Peel Commission. As the Jewish population grew, and the hostilities also grew, the feeling was that this one-state solution wouldn't work. Even so, the superpowers were advocating it as late as 1946. So now we get to the next crucial question: why did they change position? Why not leave it an Arab-majority state?

Well, from a political perspective the Jewish and Zionist population in the United States had won the hearts and minds of the American people. They believed that the plight of the Jews, especially after the Holocaust, could only be solved if they had a state. And the 5 million Jews in the United States, mostly located in battleground states like New York (still lost to Dewey in 1948) and California (won by less than 20,000 votes), was pretty important to the American politician. The Jews were also traditionally Democrats, meaning Truman had incentive to appease his base and get them to turn out in those key battleground states. Jews were also wealthy and politically organized and heavily motivated to lobby and donate to political campaigns, meaning politically it was better for the Democrats at home, since the people supported an Israeli state.

On October 4, 1946, Truman decided Jewish statehood and partition was better. Why did he make this about-face now, and not sooner? Well, part of it was what he said, that it "...would command the support of public opinion in the United States". The coming midterm elections probably inspired that. Not that it helped much, Truman still lost the Senate and the House. Still, Truman had already made his decision. He figured partition was the only way out, he had friends among the Zionists (notably Chaim Weizmann), and he saw them as culturally similar to the West. This would play a theme in the partition negotiations too. When the Zionists almost lost the Negev to the Arabs when dividing up the land, Weizmann called Truman and asked him to intervene: Truman saw the Negev as "the frontier" the US had, and kept the bulk of it for the Jews.

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