On April 15, 1943, for example, Churchill decided that a cross-Channel invasion would have to be postponed until 1944. In the course of the same day he also had to start digesting the implications of two ominous pieces of news - he learned for the first time that the Germans were developing long-range rockets (the future V1 and V2), and that a mass grave of Polish officers had been discovered in Russia, in the Katyn forest, with a strong suspicion that they had been killed on Stalin's orders.

The fall of Singapore, El Alamein, Anzio, D-day, the bombing of Dresden, the civil war in Greece - a mere roll-call of the major episodes with which Mr. Gilbert has to deal would take up the rest of this review. And almost all of them are episodes that can now be seen as far more complex than they once appeared.

While every aspect of the story has its interest, the episodes that cast the longest shadows are those involving relations with the Soviet Union. They defy easy summary, but a few points are worth stressing.

Churchill had been a fierce opponent of the Soviet regime from the beginning, and it would be absurd to suppose that he simply abandoned his long-held convictions when Hitler invaded Russia. Even in the early stages of ''Road to Victory'' there are minor instances of his mistrust of Soviet postwar intentions, and the Russian refusal to aid the Warsaw uprising of 1944 - even though the Red Army was already on the banks of the Vistula - fully confirmed him in his suspicions. Unfortunately it also provoked a disagreement with the United States, since Roosevelt rejected his suggestion that American and British planes should drop supplies over Warsaw and then refuel on Soviet landing-strips, risking Stalin's displeasure if necessary.

Some five months later he went to Yalta, where he briefly persuaded himself that the Russians would honor their pledges, particularly their promise to hold free elections in Poland. But how much of a choice did he have? A letter he sent shortly afterward to the Prime Minister of New Zealand spells out what had probably been his true reading of the situation all along. Britain and the British Commonwealth, he wrote, were much weaker militarily than Russia, and: ''We cannot go further in helping Poland than the United States is willing or can be persuaded to go. We have therefore to do the best we can.''