The source has no mention of the USA joining the war- but because the source was written by the USA's allies at the time then it is no surprise.
There was the whole fear of communism spreading but also the oppertunity to undermine communism.
The USA siezed for an oppertunity to make communism look weak and took it very willingly, especially if it meant that they could compete indirectly with the USSR.
The timing of when this text was written is vital - the USA were still very cold towards the USSR because of the whole idea of comunism spreading; the dominoe effect. China had just turned communist and this made the USA very scared that the dominoe effect may actually happen. There was no mention of rivalry between the USA and USSR.
Basically, the objective of this propaganda is to make the Britains believe that Stalin and comunism are the reasons for the beginning of the Korean war.
However, the source does not tell us that Syngman Rhee (from the South- USA) provoked the other side (North- USSR) into attacking them by vicious taunts and boastings. This set the trigger for North Korea to go to war with the South.
The USSR only had two reasons to get involed with the war. Their most important reason was so they could spread communism, but the second was so they could fight the USA without an actual 'hot war' - they could have a chance to be indirect.
I agree somewhat with the source but not to the full extent of the Korean war being Stalin's fault. I mainly disagree though because the source is very mis-leading and is not telling us the whole truth. Britain is anti-communist, they still fear a nuclear war and they have a fear of communism spreading- just like with the USA.