As
@nbcman says the likes of Little Boy and Fat Man lack the power to destroy a major European city or Field Army. They're horrific and high effective weapons but would need to be employed in large-ish numbers in a short time-frame to be considered war winners and would likely result in Britain being drenched in nerve gas in retaliation.
Delay it until 1948, as long as Germany does not posses a Nuclear Weapon. It isn't hard. The US had two bombs in 1945, they had a bunch more in 1949. If they use it on Germany or the USSR, one at least, it could scare of one of the nations. Drop it on a few important places around the same time: Berlin, Ruhr Industry, Hamburg/Bremen/Lubeck ports, a some airfields near major cities and you can already cripple the ability of the said nation to respond properly. Especially if the said nation has not nuclear weapons (yet).
If Germany is hit, the USSR will back off, or feel some of the heath until they back off. Considering the USA had 299 nukes in 1950, which would be around 150-200 in 1948 (my guess), it seems more than enough to cripple one of those nations and scare of the other.
But the same works for the other side, to remind you, I said the alliance that has nukes first. If Germany has it first they can and could use it against the UK. The US is a harder situation considering how far it is and how much stronger their navy is compared to the German-Soviet Navies.