Assume, for sake of argument, that America waits until the first successful Soviet nuclear test on August 29th, 1949 to act.
At that time, America responds by dropping 6 nuclear bombs, one of the Kremlin, and one on each of the 5 most important sites for the Soviet nuclear program.
The Soviets are then given 7 days to surrender unconditionally, or the second wave will begin. During this time additional America forces being moving towards the West/East German border from the homeland (again, assuming the administration didn't do anything differently before August 29th, 1949).
Assuming, for sake of argument, they do not surrender unconditionally, a second wave of 36 nuclear bombs will be dropped on major concentrations of Soviet industry.
The Soviets are then given 7 days to surrender unconditionally.
Assuming, for sake of argument, they do not surrender unconditionally, America moves to a full war footing, including a draft. Conventional forces begin probing attacks into East Germany, with ~50 nuclear bombs allocated to this theater for use in the first 3 months of operations to target concentrations of enemy forces, logistics hubs, etc, as they become available, prioritizing minimizing civilian German casualties as part of an agreement with the West Germans to get them to jointly mobilize.
In the mean time, another ~50 nuclear bombs are dropped on targets of opportunity inside Russia. Aerial campaigns drop leaflets over non-Russian territories of the USSR and its satellite states telling them that no nuclear devices will be dropped on their territory if there is a largescale effort by their soldiers to resist serving the Red Army in the conflict with the Americans, with ~10 nuclear devices allocated to making an example of one or two nations who are unusually loyal to the Red Army.
This takes us to approximately the end of 1949, by which time up to 150 nuclear bombs have been used. Russia's industrial base will have been destroyed, most of its non-Soviet and some of its Soviet satellites will have rebelled, its leadership will have been decimated and the survivors unable to spend any time in a major population center, and its defeat in a conventional war will be just a matter of time as American, German, and other allied forces press eastwards.
For reference, America possessed 110 nuclear bombs at the end of 1948, 235 by the end of 1949 (
journals.sagepub.com/doi/pdf…), and 369 by the end of 1950. This would leave 85 in the arsenal even if all allocated above were used, with another 134 coming in 1950. In our timeline, the Soviets produced 1 nuclear bomb in 1949 (used in a test) and only 5 in 1950, even without their nuclear sites and industrial base being bombed.
After the defeat of the USSR, the industrial and monetary base underpinning the spread of global communism will have been removed, making Communist insurgencies elsewhere much much easier to contain, and the hot war with a Communist power would flush many Communist sympathizers out of their positions of power in the US government, media, and academia, making the post-war American political equilibrium more stably anti-Communist.
"The China Question" would be subject of much discussion, but cut off from global trade, without a Soviet start to its nuclear program, and aware of the consequences of trying to develop nuclear weapons, China could be contained indefinitely.