Winston Churchill's 1895 journey to Cuba was far more formative than has been previously understood, a new book suggests, saying Cuba is where the 20-year-old junior officer discovered he had courage under fire and confirmed his own sense of greatness.

In Churchill Comes of Age: Cuba 1895, author Hal Klepak traces characteristics that made Churchill famous to his 18 days in Cuba, where he was on loan from the British army to observe colonial Spain's defence against independence fighters.

History previously recorded that Churchill saw combat in Cuba and discovered the siesta, which would later help him keep long hours as British prime minister during the Second World War. But Klepak, a former Canadian military officer, argues previous works overlooked how influential the Cuban venture was, including the months of manoeuvring Churchill needed to land his assignment.

With his Cuba experience he became a war correspondent, political analyst, strategist and liaison with a foreign army, all for the first time. His writings started to show legendary humour. He discovered rum and Cuban cigars' breadth and quality.

Inspired by observations from local historian Lourdes Mendez, Klepak believes he became the first to scrutinise and cross-check the Cuban, British and Spanish archives, discovering for example that Churchill was fired upon by no less than Antonio Maceo and Maximo Gomez, two of Cuba's greatest independence leaders. "Very quickly when I looked at it from a historical perspective it was pretty obvious that this was an amazing story which for some reason had never been told," Klepak said.

Upon graduating from Sandhurst military academy, the young second lieutenant desperately wanted a war to test his courage and make a name for himself. His father had just died, and he told his American-born mother about his plans to go to Cuba rather than ask her permission. Once there, he found he passed the test, not shirking when the Spanish army columns he accompanied were attacked by Cuban independence fighters.

"It's his baptism of fire and of course it's particularly curious because it's also his 21st birthday, which is why I choose to call the book Comes of Age because he literally comes of age and in all these dozens of senses he comes of age as well," Klepak said.