From World War II at Sea, by Craig L. Symonds (Oxford U. Press, 2018), Kindle pp. 208-209:
None of the celebrating pilots aboard the six Japanese carriers could possibly have known that just the day before, on the other side of the world, Marshal Georgy Zhukov had directed a counterattack of half a million Russian soldiers against German forces outside Moscow. Before the winter was over, the Russians would push the Germans some two hundred miles to the west. Japan had joined the war at almost the precise moment that the German juggernaut was exposed as vulnerable after all.
However tactically successful, the Japanese raid on Pearl Harbor stands alongside Hitler’s invasion of the Soviet Union as one of the most reckless and irresponsible decisions in the history of warfare, and along with the Russian counterattack outside Moscow marked a decisive turning point in the Second World War. It brought the United States and its vast industrial resources fully into the conflict and galvanized American public opinion in such a way as to ensure not only an eventual Allied triumph, but what Roosevelt in his December 8 speech to Congress called “absolute victory.”
In view of that, it is easy to overlook the fact that the raid on Pearl Harbor was only one element of Japan’s grand strategy. In fact, the Japanese began to seize the southern resource area—the actual target of all their planning—at virtually the same moment their aircraft were crippling the American battle fleet. On December 4 and 5, as Nagumo turned his carriers to the southeast (and Zhukov assembled his divisions outside Moscow), Japanese invasion flotillas left Hainan Island, in the South China Sea, and Cam Ranh Bay, in Indochina, to steam southward into the Gulf of Siam. Even as the first plane lifted off from Nagumo’s carriers, a Japanese invasion force of twenty-one transports, escorted by a light cruiser and four destroyers, began landing soldiers on the north coast of British Malaya at Kota Bharu, just below the border with Thailand (formerly Siam). Ninety minutes later (as Fuchida’s planes were lining up for their attack run on Battleship Row), a second invasion force of twenty-two transports, escorted by a battleship and five cruisers plus seven destroyers, began landing soldiers at Singora Beach inside Siam, 130 miles up the Kra Peninsula.