We "didn't know" the hazards of smoking until 50 years later .....
While there were cigarettes in rations in WWI and WWII, the widespread use of cigarettes was more prominent after WWII and by the 50's shortly after, is when the links to cancer were established and further promoted and established - basically a decade or less after widespread use of cigarettes by the population of the US
wiki:
The widespread smoking of cigarettes in the Western world is largely a 20th-century phenomenon. At the start of the 20th century, the per capita annual consumption in the USA was 54 cigarettes (with less than 0.5% of the population smoking more than 100 cigarettes per year), and consumption there peaked at 4,259 per capita in 1965. At that time, about 50% of men and 33% of women smoked (defined as smoking more than 100 cigarettes per year). "tobacco Use, United States 1990-1999". Oncology (Williston Park) 13 (12). December 1999.
In 1950, Richard Doll published research in the British Medical Journal showing a close link between smoking and lung cancer. Four years later, in 1954, the British Doctors Study, a study of some 40,000 doctors over 20 years, confirmed the suggestion, based on which the government issued advice that smoking and lung cancer rates were related.