After an administrative landing at Wonsan on 26 October, X Corps, now including the US 3rd Infantry Division, advanced inland northwest towards the Yalu River with the ROK I Corps made up of two ROK Divisions in the far north or right flank. The US 7th Infantry Division was in the center and the US 1st Marine Division on the southern or left flank of the X Corps attack. The 3rd Infantry Division was initially in reserve. As elements of ROK I Corps and 7th Infantry Division closed on the Manchurian border, the 1st Marine Division moved into the Chosin Reservoir (Changjin Ho). The Marines were on both left and right sides of the Changjin reservoir. Regimental Combat Team 31 (RCT 31 also known as Task Force Maclean/Task Force Faith) of the 7th Infantry Division replaced the 5th Marine Regiment on the east side of the reservoir in a piecemeal fashion with only two of its three maneuver battalions in place before heavy engagement with the enemy commenced. X Corps was strung out along many miles in sub-freezing temperatures with the ROK troops and the 7th Infantry Division to the north in contact with PVA forces. RCT 31 was too far from its parent Division for support and without organic tank support and its third maneuver element; it was decimated by the onslaught of the PVA. The 1st Marine Division fared better and with remnants of RCT 31, Army Engineers and X Corps support personnel, began its move to the sea moving through elements of the 3rd Infantry Division (Task Force Dog from the 7th Infantry Regiment, and a reinforced battalion of the 65th Infantry Regiment) who provided flank and rear guard cover for the withdrawing units. The 7th Infantry Division in the center and ROK I Corps on the right flank also began withdrawing to the Hungnam beachhead. The Marines withdrew through the 3d Infantry Division with intermittent contact with PVA forces up to Sudong. The extreme temperatures during this period caused the majority of the casualties for X Corps. The Marines managed to reach the safety of Hungnam first, where the 3rd and 7th Infantry Divisions and I ROK Corps provided perimeter defense. The Marines were evacuated by the middle of December, followed by the 7th Infantry Division, I ROK Corps and the last of the X Corps' elements. The 3d Infantry Division was last to leave the beach and evacuated on 24 December 1950.