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The martial art of Kali (aka Escrima, Arnis) is said to have protected the Philippines from various foes: Spain, United States, Japan and others. The Southern Philippines, particularly the island of Mindanao is noted in history as being particularly problematic for enemies. While there is undeniable history surrounding these basic facts, there are some surrounding rumors that should be addressed.
LeathernecksI've had multiple Kali / Escrima / Arnis folks tell me that the origin of the term "Leatherneck", a slang term for a member of the United States Marines, comes from a band of leather worn by Marines around their necks to prevent cuts to the throat. The claim is that Moro and other Filipino warriors around or after the Philippine–American War were so routinely killing American Marines by cutting their throats that they adopted the leather collar to prevent this. Many sources (1,2,3) point to other reasons much earlier in history - all the way back to 1776. Reasons being "it prevented the neck movement necessary for sighting along a barrel" and "to protect the neck against cutlass slashes and to hold the head erect in proper military bearing". Most also say that the leather collar was abandoned before the Philippine–American War started and I have read nothing saying that it was ever resurrected. It would not surprise me if the military readopted this practice either formally or informally and it is not documented. It's also likely that some soldiers may have individually added this to their armor - practice still going on today. Regardless of whether it was or not, the evidence seems to predate US Marines in the Philippines.
.45 Colt (aka .45 ACP, .45 Auto)
1) 3 inch 12 ga magnum shotgun shell
2) AA battery (for size comparison)
3) .454 Casull
4) .45 Winchester Magnum
5) .44 Remington Magnum
6) .357 Magnum
7) .38 Special
8) .45 ACP
9) .38 Super
10) 9 mm Luger
11) .32 ACP
12) .22 LR
ConclusionThe term "leatherneck" seems to come from an earlier time, but it isn't necessarily true that American soldiers did not wear leather collars during the occupation of the Philippines. The stories of the time before the Colt .45 (.38 calibers not strong enough to stop a drugged or determined Filipino) certainly back up why soldiers would wear it. This is probably why the stories are often told together.
History seems to point in favor of the Colt .45 legend.