In a two-plus hour Republican presidential debate focused on "keeping America safe," candidates mentioned guns just once — when Jeb Bush said, "America is under the gun to lead the free world, to protect our civilized way of life."

But guns are one of the biggest security threats to Americans. The research strongly indicates that America's easy access and abundance of firearms contributed to the deaths of nearly 34,000 people in 2013, including more than 11,200 due to homicides. (Terrorism killed fewer than 75 Americans annually between 2002 and 2011.)

This is not to say that Americans should not care about terrorism and only care about gun violence. Rather, it shows that for all our (justifiable) time spent seriously debating how to confront terrorism, many of America's presidential candidates aren't willing to even discuss how to deal with America's gun problem, even though it takes thousands of lives each year.

And let's be clear: America's gun problem is very unique in the developed world.

America's unique gun problem

No other developed country in the world has anywhere near the same rate of gun violence as America. The US has nearly six times the gun homicide rate as Canada, more than seven times as Sweden, and nearly 16 times as Germany, according to UN data compiled by the Guardian. (These gun deaths are a big reason America has a much higher overall homicide rate, which includes non-gun deaths, than other developed nations.)

In fact, no other developed country comes close to the levels of gun violence, including suicides, that America has, as this chart from Tewksbury Lab shows:

The correlation this chart demonstrates — more guns mean more gun deaths — has been backed by a lot of research. Whether at the state or country level, reviews of the evidence by the Harvard School of Public Health's Injury Control Research Center have consistently found that places with more guns have more deaths after controlling for variables like socioeconomic factors and other crime. "Within the United States, a wide array of empirical evidence indicates that more guns in a community leads to more homicide," David Hemenway, the Injury Control Research Center's director, wrote in Private Guns, Public Health.

This is widely believed by experts to be the consequence of America's relaxed policy approach to and culture of guns: Making more guns more accessible means more guns, and more guns mean more gun deaths. Researchers have found this is true not just with gun homicides, but also with suicides, domestic violence, and even violence against police.

Here's one chart, from a 2007 study by Harvard School of Public Health researchers, showing the correlation between statewide firearm homicide victimization rates and household gun ownership after controlling for robbery rates:

At the same time, other developed nations have had some big successes curtailing gun violence by reducing the number of guns. After a 1996 mass shooting in Port Arthur, Australia, killed 35 people and wounded 23 more, lawmakers passed new restrictions on guns and imposed a mandatory buyback program that essentially confiscated people's guns, seizing at least 650,000 firearms.

According to one review of the evidence by Harvard researchers. Australia's firearm homicide rate dropped by about 42 percent in the seven years after the law passed, and its firearm suicide rate fell by 57 percent. Although it's hard to gauge how much of this was driven by the buyback program, researchers argue it likely played some role: "First, the drop in firearm deaths was largest among the type of firearms most affected by the buyback. Second, firearm deaths in states with higher buyback rates per capita fell proportionately more than in states with lower buyback rates."

Still, similar measures would be very difficult to pass in America, a nation in which gun culture and ownership are deeply ingrained. Republicans' unwillingness to even discuss the issue is a good example of that.

Watch: America's gun problem, explained

Correction: Due to a transcript error, this article originally suggested there were three mentions of guns. There was only one.