https://www.gatestoneinstitute.org/19502/do-babies-still-win-wars
China, Russia and Iran spend almost twice as much on the military as they do on the health and well-being of their citizens. Their military expenditure as a percentage of GDP is more than twice the average of OECD countries, while their healthcare spending as a percentage of GDP is less than half of the OECD average.
Since the Chinese sage Sun Tzu authored The Art of War around 2,500 years ago, almost all writers on military affairs have asserted that a rapid rate of population growth is the sine qua non for a nation’s decision to go to war.
More recently, this theory was elaborated by the Swiss mercenary, General Antoine-Henri Jomini, in a series of books that have been “must reads” in most military academies since the 19th century. Echoes of the same theory are present in On War, the military “bible” written by Carl von Clausewitz, the Prussian general and military historian, in the 19th century.
Thus, the Indo-European tribes left their ancestral homeland in Central Asia because they could no longer feed a rapidly growing population. Seeking more fertile land and pastures for their herds, they embarked on invasions to their west and south to find the resources they coveted.
The ancient Romans and Persians were also pushed towards conquest because of high birth rates that made the seizure of new lands a life-and-death necessity.
The Arab invasion of the Persian and Byzantine Empires in the Middle East and Asia Minor also came when the Arabian Peninsula could no longer feed its rising population. The same analysis has been applied to the Mongol invasion that brought hungry tribes from far away Mongolia to the heart of Europe and the Middle East.
Rapid population growth was also a key factor in pushing the British on a path of empire-building from the 16th century onwards. In fact, the ability to export large numbers of peoples to new lands is often cited as one of the key factors in empire-building since time immemorial.
Finally, the Nazis claimed that post-Weimar Germany needed to invade Europe in search of “Lebensraum” (living space) to secure agricultural and energy resources needed to be a big power.