World Population Halved

[Vega]

What if in 2017, instead of a world population of over 7.5 billion people, the total was just 3.5 billion people. This would make it roughly the same number as the 1970s, though the 70s and past decades would be lower in this alternate history as well.

How could this be achieved?

[Kingpoleon]

1914-1922: World War One
Germany, Italy, Austria-Hungary, Ottoman Empire(Joined After 1914: America, Spain, Bulgaria, Romania, the Netherlands, China) vs. Serbia, Russia, France(J. A. 1914: United Kingdom, Belgium, Greece, Albania, Portugal, Denmark, Japan)
Grand Alliance/Berlin Alliance Victory

1931-1936: World War Two
Imperial Germany, Hungary-Slovakia, the Netherlands, Napoleonic France, United States, Imperial Russia, Royal Yugoslavia, China vs. Socialist Spain, Socialist United Kingdom, Portugal, Italy, Austria-Bohemia, Turkey, Ukraine, Sweden, Spain, Mexico, Japan
Socialist Victory

1957-1968: World War Three
Germany, Yugoslavia, France, Russia, United States, Hungary, Russia, Republic of China, Argentina, FRCA vs. Japan, Italy, Spain, United Kingdom, Mexico, Canada, Brazil, Persia, Turkey, India
Republic/Royalist Victory over Fascists

Afterwards, Japan, India, China, and Russia were involved in many wars, and smallpox was repeatedly weaponized. In the Americas, the Caribbean Union, the USA, the FRCA, Brazil, and Argentina fought Mexico, Canada, Chile, Peru-Bolivia, Venezuela, and Colombia. In Europe, France, Germany, Yugoslavia, and Romania maintained a strong alliance against socialists and reactionary/"Mussolinian" movements. Strasserism and Röhmism on several occasions managed to ally the fascism, communism, socialism, and anarchism movements. Eventually, however, the divisions were always exploited. In Africa, Egypt-Sudan, Ethiopia, Nigeria, Ost Afrika, and Morocco maintained power. In the Middle East, the reformed Kingdom of Persia, the Republic of Turkey, the People's Republic of Arabia, the Republic of Syria and Jordan, and Israel had a comparatively peaceful time, though they, too, greatly suffered from smallpox and the Soldier's Flu.

[Boston Bread]

The Cold War turning into nuclear war is the most straightforward way.

[The Mikado]

If your point of departure is early enough, your answer could just be that the Green Revolution never happened. That's a lot easier and less pleasant than disaster or nuclear war.

[○∙◄☻¥tπ[╪AV┼cVê└]

Many decades ago, the Catholic,  Muslim, and Hindu churches start distributing free contraceptives, and warn about the dangers of overpopulation. Areas like south Asia and Africa have many fewer people and a higher standard of living that encourages fewer births.

Also, maybe a major land war involving China.