Why can't Europe defend itself?

[Vuk]

The rates of obesity and general ill-health are far greater in the USA than anywhere in Europe.

Americans mentally have never had hardship in war since probably the American Civil War, and even then it was sporadic.

Most weaponry does not require a 18 stone wall of muscle to use. AKs can even be used by children effectively. I think one of the German Emperors was the last person to try to get giants to fight as a unit (the father of Peter the Great). They were useless and slaughtered by cannon and shot. Things have advanced since then.

Although as I say above, the wide neck is more usually going to be encircling a vast amount of fat.

Do you really think that strength and physical fitness are not important to combat? Combat tests your body's limits to the max, and the limits of American bodies are a lot higher than that of European bodies. The average European probably could not even carry a wounded comrade off the field.
Just as important is the work on the homefront to support the war effort. Large size and strength is not only a product of hard work, but essential for the type of work that needs to be done in the case of a sustained war.
The Russians, Chinese, Indians, and Americans are tough people who can cope with hardships, but Europeans who do not even work half of the year would shrivel up and die.
I hate hurt your feelings Rory, but size does matter.