Civil Engineer Corps

U.S. Naval insignia, brass subdued per USMC regulation for CEC in CBs transferred to the Corps.

ChCarp. W. H. Achenson (CEC) awarded Silver Star for UDT 1 actions

During WWII the Seabees had a number of battalions transferred to the Marine Corps.[11][12][13] Those battalions were then given USMC designations and the men were given standard Marine Corps issue in addition to their dress naval uniform. For CEC the standard gold and silver officer corps insignia was replaced by a brass subdued one on the garrison hat. The battalions involved were the 18th, 19th, 25th, 53rd and 121st.(see 17th Marine Regiment, 18th Marine Regiment, 19th Marine Regiment, and 20th Marine Regiment) The 31st and 133rd CBs were issued USMC fatigues and attached to the shore parties of the 4th Marine Division and 5th Marine Division for Iwo Jima. The CEC involved would have worn the subdued insignia also. Other battalions were tasked with Marine Corps shore party assignments both prior to and post-Iwo Jima.

Tasked as combat engineers, the CEC of the 18th and 121st CBs designed a detachable ramp mounted on a LVT-2.[5] Its purpose was to enable the Marines to land on Tinian's beaches bordered by coral embankments up to 15 feet high. Ten LVTs were modified using iron beams salvaged from a sugar factory on Saipan.[5] The commanding General Harry Schmidt was skeptical of the design. He ordered that a vehicle test one, a hundred times, before he would use it in combat. The ramps not only stood up, but they allowed the Marines to land where there were no defenses as a landing there had been thought impossible.[5] The astonished Japanese were overwhelmed and outflanked due to the ramps. The LVTs were nicknamed "doodlebugs".

• Guam: 3rd Marine Regiment, Cmdr. Wehlen CEC 3rd Battalion 19th Marines / Naval Construction Battalion 25 beaches Red 1 and Red 2
• Bougainville: 3rd Marine Division Cmdr. Brockenbrough CEC 71st CB with detachments from the 25th, 53rd, and the 75th CBs (and as well as the Marines).[3] beaches: Blue 1,2,3 Yellow 1,2,3,4 Green 1,2 Red 1,2,3
• Iwo Jima: 23rd Marine Regiment Cmdr. Raymond P. Murphy CEC, 133rd CB beaches Yellow 1 and Yellow 2.

Operational Naval Demolition Unit No. 1. was the very first USN "demolitions" unit. In early May 1943, a two-phase "Naval Demolition Project" was directed by the Chief of Naval Operations "to meet a present and urgent requirement". The first phase began at Amphibious Training Base (ATB) Solomons, Maryland with the establishment of Operational Naval Demolition Unit No. 1. Six Officers and eighteen enlisted men reported from NTC Camp Peary dynamiting and demolition school, for a four-week course. Those Seabees, led by Lt. Fred Wise CEC, were immediately sent to participate in the invasion of Sicily.[14][15] When the unit returned to Camp Peary most of the men were assigned to the new Naval Combat Demolition Units being formed there.

Naval Combat Demolition Units were led by junior CEC officers.[16] There were over 200 NCDUs formed with all but five being requisitioned for the UDTs.

Lt. Crist (CEC), Lt. Cmdr. Kaufmann and Lt. Carberry right to left at awards ceremony

V Amphibious Corps had identified coral as an issue for Amphibious landings in the Pacific and determined Naval Constructions Battalions had the only people with any experience with the material. Lt. Thomas C. Crist CEC, from NCB 10 was in Honolulu from Canton Island where he had been involved in a lagoon coral head clearance project.[17] His being in Pearl Harbor turned out to be pivotal in UDT history. Admiral Turner and V Amphibious Corps were interested in dealing with coral and had identified the Seabees as the only people with any applicable knowledge. The Admirals staff learned of Lt. Crist's presence in Pearl Harbor and ordered him to report. The Admiral commissioned Lt. Crist with developing a method to blast coral under combat conditions and staging qualified men in Pearl to form a unit for that task. Lt. Crist had staged 30 officers and 150 enlisted from the 7th Naval Construction Regiment when the disaster at Tarawa happened. With Kwajalein the next operation, Lt. Crist's 180 men were used to form UDT 1 and UDT 2. Cmdr. E. D. Brewster (CEC) was selected to command UDT 1 and Lt Crist was picked for UDT 2. That did not last as Admiral Connelly wanted a commander with combat experience. So, Lt. Crist was made ops officer for team 2. At Kwajalein Ensign L. Leuhrs and Carp. W. Acheson CEC anticipated that they may not to get the intel Admiral Turner wanted just paddling a dinghy and wore swim trunks under their fatigues. They decided to strip down and go in the water in broad daylight on a hostile beach to get what the Admiral wanted. Doing that changed the UDT mission model and made them the predecessors of the Navy's special ops. Upon returning to Hawaii Lt. Crist was named as the first Training Officer of the UDT program. He was in the position only a short time when he was selected as commander of UDT 3. For the Marianas operations of Kwajelein, Roi-Namur, Siapan, Tinian, Eniwetok, and Guam. Admiral Turner recommended sixty silver stars and over three hundred bronze stars with Vs for the Seabees and others of UDTs 1-7,[18] which was unprecedented in U.S. Naval and Marine Corps history.[18] For UDTs 5 and 7 every officer received a silver star and all the enlisted received bronze stars with Vs for Operation Forager (Tinian).[14] For UDTs 3 and 4 every officer received a silver star and all the enlisted received bronze stars with Vs for Operation Forager (Guam).[14] Admiral Richard Lansing Conolly felt the commanders of teams 3 and 4 (Lt. Crist and Lt. W.G. Carberry) should have received Navy Crosses.[14] When UDT 3 returned from Leyte in November 1944 it became the training instructors of the Maui school and Lt. Crist was made base Training Officer again.[19] The team would remain in these jobs until April 1945 when it was sent to Fort Pierce to do the same job there. Lt. Crist had been promoted to Lt. Cmdr. and was sent back to Hawaii but his Team 3 Seabees would train teams 12–22.[19]

Diving masks were not common in 1944 and a few men had tried using goggles at Kwajalein.[20] They were a rare item in the Hawaiian sports stores so Lt. Crist and Seabee Chief Howard Roeder and put in a request to the Supply Corps for them.[20] Fortuitously, a diving mask ad was spotted in a magazine. That prompted a priority dispatch to the States appropriating the store's entire stock.[20]

In 1944 the Navy created an unheralded program to dredge harbors to increase accessibility and stevedoring productivity at advance bases. The 301st CB was created for the job and given two ex-NCDU (CEC) and two ex-UDT (CEC) to assist. Between them they had three Silver stars and a Bronze star for valor.

During WWII fifteen CEC were taken as prisoners of war.[21] All were in the Pacific and all were taken at the onset of hostilities at Cavite, Philippines, Wake, and Guam.[21] Six would die: one executed, two from friendly fire, and three from mal-treatment.[21] One POW, Lt. Jerry Steward CEC, received the Navy Cross, Purple Heart with three gold stars, Army Distinguished Unit Badge with Oak leaf cluster, Philippine Distinguished Service Star and was the most decorated CEC officer of WWII. Postwar he retired as a Rear Admiral.[21]