Billy Mitchell thinking airplanes would be crucial in war after WWI but didn't have much support comes to mind. He wanted to create the Air Force (basically) but the Navy thought that planes were terrible compared to ships and couldn't sink a ship so Mitchell proved them wrong by sinking a captured German battleship. It is now obviously how important planes are in the military.

Another that comes to mind is cryptology, before Pearl Harbor there was very minimal manpower and effort dedicated to breaking the Japanese Naval Code (JN-25) and even though it was pretty clear an attack was coming (since the US had broken the diplomatic code, PURPLE) they didn't exactly know where or when. After Pearl Harbor more manpower and money was spent on breaking JN-25 which led to the US winning the battle of midway and sinking of 4 Japanese carriers which basically turned the war around and made the US realize how important code-breaking was.

Not sure if this qualifies as an online presence but since it kind of fits in with your question, specifically with STUXNET the US managed to screw up Uranium enriching in Iran by infiltrating underground bunkers and varying the speed of the centrifuges so they would break down more while telling the operators that the machines were functioning normally causing major breakdowns and delays and ruined about a fifth of Iran's centrifuges. This also got into many powerplants and oil pipelines and had the ability to shut them down and destroy them. The origin is also unknown, although almost certainly Israel and/or the US.