the north invaded the south

that sparked the war

the US never declared war on the north, the US was merely defending its allied interest of South Korea. To be honest, it was a UN mission. There were many countries that committed troops to defend south korea. The advance into North Korea by US and allied forces was done under a UN operation. Everyone just likes to see it as a US war. The UN council was the one who recommend member states support the south

the truce you are speaking of is between the north and south ... not the north and the US.

and if you want to get technical, it was never a war rather a police action ... which allowed the US to support the south without a declaration of war. we never were actually at war with the north, we were defending allies as part of a UN mandate.

edit:

just for arguments sake these are the countries that sent troops supplies and equipment to south koreas aid:

Australia
Belgium
canada
columbia
denmark
ethiopia
france
greece
india
italy
luxembourg
netherlands
new zealand
norway
phillipines
south africa
sweden
thailand
turkey
UK

and the reason you DONT hear about alot of these countries is that most of them operated under US command with US weapons and equipment. the thai's were actually some of the first in country but were assigned to a US regiment same with turkey.