Hegemony. It's hard to have serious inter-state conflicts in the neighborhood of an unquestioned hegemon.
Pax Americana is most noticeable in the Americas, far from the rival powers of Russia and China. If the U.S. occupies or regime-changes a small country in this area, it tends to be quick, and the death toll is naturally less than an evenly-matched conflict that drags out for years.
GDP rankings are suggestive of the amount of money and industry we could afford to throw at a war effort. Countries like Brazil, Mexico, Canada are nowhere near the U.S. here. The great powers of Europe have long been in the same order of magnitude. Their
complex dance shows the gains of development and the losses of war.
Latin America's turbulent 19th century had the wars of independence and the Triple Alliance against Paraguay, setting the borders close to what they are today. The American Civil War was the deadliest war in the 19th-century Americas, and practically all major conflicts in the 20th-century Americas have been civil wars. The 1930s Chaco War was between the two landlocked countries, so rather far from the US Navy.
There's a catch with direct comparisons: the Americas have held about
1/7 of the world's people since the mid-20th-century. They surpassed Europe in the 1990s and only recently hit 1 billion. In the 19th-century, they were fast-growing but not yet 10% of the world's population.
The Americas punch above their weight in
homicide, whose annual death toll would extend into war-like numbers. The Mexican Drug War even gets included on many lists of ongoing wars.
