Source A says that the communist forces were the ones who started using military force to invade South Korea in order to spread communism. 

This is seen in how Source A states that, “The attack upon Korea makes it plain beyond all doubt that communism has passed beyond the use of subversion to conquer independent nations and will now use armed invasion and war”.

In contrast, Source B says that USA was the initiator of the Korean War by invading North Korea first through using military forces.

This is seen in how Source B states that, “The American imperialists furiously carried out the war project in 1950. The American invaders who had been preparing the war for a long time, alongside their puppets, finally initiated the war on June 25^th”.

Thus, as 'support' sources, A, B and E are largely unreliable and this makes it likelier that domestic issues were the main factor that motivated the USA to become involved in the Korean War.

Overall, the ‘support’ set of sources are preferred. This is because the reliability and utility of the ‘challenge’ set is undermined by key issues such as contextual knowledge which differs from the claims of sources A,B and E. Where in contrast the key strength being contextual knowledge had corroborated the claims of the ’support’ set thus enhancing the reliability and utility of the preferred set of sources.

Needed to highlight SPECIFICALLY the key ideas that undermined the less preferred set of sources and why this was the case, and the key ideas the enhanced the overall reliability of the preferred set of sources and why.

able to take over mainland China attributed to Mao Zedong and his communist party

coming into power in 1949. Thus, Truman was facing much backlash at home as he was seen to have done nothing to save Chiang Kai Shek’s China.

Source B offers a less credible perspective on the issue of why the US got involved in the Korean War because it is an excerpt from a North Korean textbook published in 1999, and the author had the aim of forming an image that the US were the aggressors in the Korean War, that threatened the peace of North Korea, in order to ensure the continuous loyalty of the North Korean people to the its government. Also, contextual knowledge contradicts Source B's claim that the US was the initiator of the Korean War because it was actually the North Koreans who started the invasion on  25th June 1950 following the Chinese Communist Party’s Victory in China in October 1949, in order to expand their sphere of influence by attempting to reunify Korea under communism. [After undermining the reliability of Source B's views, you need to link back to why the source, therefore, offers a less credible perspective on whether domestic or external factors motivated the US to get involved in the Korean War.]
Source E offers a less credible perspective on the issue of why the US got involved in the Korean War because it was written by a New Zealand author writing a political cartoon which tends to be caricature rather than seeking to provide a balanced analysis Also, contextual knowledge tells us that China only became communist on 1 October 1949, Mao Zedong the leader of the Chinese communist party was more preoccupied with China’s economic reconstruction and even feared a war with the US. Even though by October 7th Mao agreed to intervene in the Korean War because of the fears of a two-front war this still undermines Source E's claim that Chinese communists were expanding rapidly because Communist China was more or less incapable and highly unlikely to meet the vast extent of reach that the New Zealand cartoon depicted due to being preoccupied with other pressing matters. [To reduce the reliability/utility of Source E, you should also focus on whether a New Zealand newspaper is likely to be able to provide a more or less valid perspective on what truly motivated the US to intervene in the Korean War.] 

Thus, as ‘challenge’ sources, A, B and E are largely unreliable and this makes it likelier that domestic issues were the main factor that motivated the USA to become involved in the Korean War.

This is seen in how Source A states that

“In these circumstances the occupation of Taiwan by Communist forces would be a direct threat to the security of the Pacific area and to United States forces performing their lawful and necessary functions in that area.”

Source A's view is reinforced by Source B that states “Upset by the fast and astonishing growth of the power of North Korea, the American invaders hastened the preparation of an aggressive war in order to destroy it in its infancy. “

Source A and B’s views are also reinforced by Source E that depicts an octopus referring to communist China extended the reach of its tentacles referring to its influence onto other territories such as  Tibet, Malaya, Indochina and Korea.

Thus, the sources challenge the hypothesis as they say that there was growing external issues that were the main factor that motivated the USA to become involved in the Korean War.

However, despite the inaccuracy of McCarthy’s claims about the 57 cases of communist spies within the US government Source C confirms that not all of McCarthy claims were inaccurate as seen by how it says that there was an exponentially growing communist threat on the global level. Also, contextual knowledge confirms McCarthy’s claims about the rising threat of the USSR and communism as seen in how contextual knowledge shows us that the USSR’s first successful atomic test in August 1949 was a demonstration of their increased military capacity, as now they had the necessary capabilities to reach newer heights in the spread of communism. 

Thus, as ‘support’ sources, C and D are largely reliable and this makes it likelier that domestic issues were the main factor that motivated the USA to become involved in the Korean War.

Source C’s view is reinforced by Source D that states “I have in my hand 57 cases of individuals who would appear to be either card carrying members or certainly loyal to the Communist Party, but who nevertheless are still helping to shape our foreign policy.”

Thus, both Sources C and D support the hypothesis as both say that there was a growing domestic issue of internal communism within the US government that would consequently result in the need for US intervention in the Korean War due to domestic reasons.

For Source A, it was a statement by President Truman on 27th June 1950. It was aimed to convince the United Nations that intervention was required in order to contain and even repel communist aggression in the south. Additionally, Truman believed in the “domino theory” whereby if Stalin went unchecked, US promises to resist communist aggression in Europe and Asia would be hollow and not credible. 

For Source B, excerpts from a North Korean textbook in 1999, published many years after the end of the Korean War. It is to paint the Americans as aggressors whom threatened the state of peace of their country, labelling them as imperialists with the intention of expanding their sphere of influence into North Korea. Aiming to instil an image of the Americans as enemies towards North Koreans. Resulting in an outcome that would convince the North Korean people to uphold communism. [How did historical context e.g., prior instances of communist aggression in Europe, influence Source A's views?]
[How did historical context e.g., written at a time far removed from the actual conflict, by a government seeking to legitimise itself through as many means as possible, influence Source B’s views?] 

 

For Source B, contextual knowledge corroborates how indeed the US believed that communist influence was growing dangerously quick, thus resulting in the second United Nations Security Council resolution being passed on 27th June 1950, supporting an international military effort to repel North Korean aggression - the alarming growth of North Korean expansion in the south. 

For Source AB, contextual knowledge also corroborates that on 25th June 1950 several divisions of elite North Korean troops, many having fought for the communist side in the Chinese civil war, crossed the border into South Korea, with the expectation of conquering the entire south in a mere three weeks.