1942 photograph of International Harvester Company's War Materials Manager donating blood. WWII was the third major event in the world that delayed blood transfusion was used in; though it was used in part of WWI and in the Spanish Civil War, major disasters did not commonly use the technique for severely injured persons until around 1940.
Blood Transfusion Recent History
Blood transfusion has been studied since long ago, but first gained major traction in the Renaissance. In 1901, Karl Landsteiner discovered how to determine blood groups, and that blood groups did not mix (which was why very early transfusions all failed). This was the first time that transfusion became a viable possibility. However, clotting from fibrin activated by the thrombin enzyme when the blood was extracted led to a complete impracticality of human-to-human transfusion, especially non-direct transfusions (when people are right next to each other and blood is taken from one and put into the other).
In 1914, an Argentinian doctor named Luis Agote performed the first successful blood transfusion, using matching blood types, and using a chemical called sodium citrate to prevent clotting. Very soon after, Belgian doctor Albert Hustin independently made the same discovery about the usefulness of sodium citrate in preventing the processes that initiated clotting. However, he used far too much of the chemical, and the patients that he treated were effectively poisoned by it.
In the following years, many more discoveries were built off of those, including finding the best concentration of sodium citrate, and the addition of glucose and refrigeration to increase shelf life (up to 14 days once citrate-glucose solution was added). In 1917, the United States successfully used previously-collected Type O blood after the Battle of Cambrai to treat soldiers suffering from blood loss and low blood pressure from shock.
Controversy
Of note, though most discoveries regarding blood transfusions were done in good faith and with good science and morality, when Dr. Richard Lewisohn discovered the optimal sodium citrate percentage for transfusion to prevent coagulation yet not toxic to the patient (0.2%), it was done with some more questionable data-gathering procedures than the others. The paper that he wrote on his work used a good deal of previous work by Luis Agote, even though Agote hadn't published formal work (he had released some preliminary/forum-type data) on the specific subject of concentration. He did not, however, credit Agote for the prior work and calculations that he'd done that Lewisohn based his tests off of. It's been argued that since it was not a formal work/primary source it did not need credit, but even back then, ALL work was credited, and it's likely that the exclusion of Agote's work was either due to nationality or racial bias. However, I could not find Agote's specific work/data/calculations, and don't know how similar it was to Lewisohn's initial assumptions, so I have no real position on the issue. Agote should be more recognized as the first successful transfusion, though, as he DID do his before Hustin (who is often credited with the first transfusion with sodium citrate).