This is a setting for Call of Cthulhu, set in the late 1940s based on one possible advancing of Lovecraft's timeline. Lovecraft pictures a world on the edge of horrors, with advancing science close to uncovering secrets man was not meant to know. This is about those horrors coming true.
The most prominent event of this timeline is the Deep War. Starting with the submarine attack on Devil's Reef near Innsmouth in 1928, a number of the advanced nations made attacks on Deep One lairs - including a British attack off Iceland in 1931 and a Japanese attack in the South Pacific in 1932. In 1933, the Deep Ones struck back for the first time.
What followed eventually escalated into all-out war. The humans made repeated strikes on underwater targets. The Deep Ones struck without warning across the globe, and swarmed over to capture many island targets. Eventually, the key allies in the fight against the Deep Ones were the U.S., Britain, Germany, and Japan. Russia and China fought individually against incursions, but did not join the Allies. A number of smaller countries that refused to join the Allies were invaded in their effort to bring the war to the enemy.
In 1942, in defiance of rationality and the laws of physics, the waves rose and inundated the New York area. Suddenly, Manhattan Island was under water, and only the skyscrapers remained above the waves. People crowded the tops of the buildings, hoping against hope for rescue, while thousands drowned beneath the deep, green sea and Deep Ones entered the city and began climbing the towers. Other horrendous monsters struck in and around the flooded areas as well. Later cities hit included Miami, Boston, New Orleans, Buenos Aires, Bristol, Brest, Cape Town, Mumbai, Tokyo, and Osaka.
The Deep One advance was halted in 1945, when the U.S. dropped the first nuclear torpedo on an undersea stronghold off Bikini Atoll. The Deep Ones withdrew from New York and Buenos Aires, held for two years, but retained most of their holdings in the Pacific.
In 1948, many of the world's islands remain enemy territory, including Hawaii and the Phillipines. Key coastal cities have been devastated by the unfathomable weapons of the enemy, and are still in the process of being rebuilt.