The US 1st Division OOB has two infantry and one artillery brigade. Each infantry brigade consists of two infantry regiments, each consisting of two battalions of infantry, and a machine gun battalion totaling eight infantry battalions and two machine gun battalions. The artillery brigade consists of three artillery regiments. Two of those have two artillery battalions and one has three battalions. The artillery brigade also has a mortar battalion. The total combat troops of the division is thus eight infantry battalions, two machine gun battalions, seven artillery battalions, and one mortar battalions. There's also division level troops but those aren't combat commands and are things like signals, the HQ unit, or units for the protection of HQ and division elements. The majority of the 1st Division is currently deployed in the US.
The US 2nd Division OOB has two of their three combat brigades stations at Ft Lewis, Washington meaning one-half to two-thirds of the US combat troops organized under the 8th Army HQ in South Korea are stationed in the US. Only a single brigade of the 2nd Division is stationed in Korea. That brigade contains a squadron of a cavalry regiment, an infantry battalion, an armor battalion, and an artillery battalion.
US forces in Korea are not significant enough to provide a huge impact to the outcome of an attempted invasion by N.Korea into S.Korea. They are also not able to play a significant part of a S.Korean invasion of N.Korea. As another poster pointed out, their presence is there only to help keep the peace by providing the bait by which the US would become reinvolved in the conflict.