At 7:53 a.m., the Japanese hit Pearl Harbor. With the entry of the U.S. into war, most of the personnel at the SCL were commissioned into the Army; Marchetti was made a Captain. Following the surprise bombing of Pearl Harbor, there was a crash program to obtain radars to protect the Panama Canal Zone from a similar attack. To detect low-flying aircraft at a range allowing sufficient warning, a high-frequency radar system for picket ships stationed 100-miles offshore was needed. Coworker Harold A. Zahl had developed a tube (VT-158) that gave up to 240 kW pulse power at 600 MHz . (This was actually four triodes and their associated circuitry tightly packaged in one glass envelope.) Marchetti led a 20-person team in adapting the SCR-268s for using this tube; the project was completed in a few weeks, with the first set installed on the M.S. Nordic. As U.S. troops began the recapture of islands in the Pacific Theater of Operations, there was an urgent need for a portable radar to provide medium-range early-warning against aircraft. Again Marchetti was assigned the task of developing a suitable system. In only a few days, Marchetti and his team converted the picket-ship radar into the AN/TPS-3, a lightweight, transportable system that could be assembled and placed into operation by a small crew in 30 minutes. Somewhat later, the set was modified as a mortar-locating radar, the AN/TPQ-3. During the war, about 900 of these early-warning and mortar-locating systems were built, including 24 sets used in the D-Day Normandy landing. For the continuing war years, Marchetti was engaged in a wide variety of radar projects. One of the largest activity was his support to the Rad Lab at MIT in developing a mobile, gun-laying, microwave system, eventually designated the SCR-584. Undoubtedly the best-known radar system of the war, this included the M-9 analog computer that set the stage for great post-war advances in this field. Attacks on England by German V-1 flying bombs began in early 1944. Several SCR-268 systems upgraded to 600 MHz were rushed to England and set up on the coast to direct the anti-aircraft guns. When first used, the radar-directed guns actually were reduced in their hits.