Not true. The United States military is the richest, most advanced, best trained, and hence, most powerful military in the world today. HOWEVER, one thing people must remember is that the US hasn't HAD to fight a conventional war to defend its own turf since WWII. Many people in the military are in to have a job with good benefits to support their families, a reason that which should not be scrutinized because even civilians are in the same exact boat.
A lot of soldiers, even loyal, patriotic ones, see the military as a job that which they don't want to die for. Which, I know it can sound cowardly; joining the military and not expecting to have your life put at risk, but civilians should also understand this as well. Soldiers are people with lives and families that not only love, but depend on them.
When it comes to the battlefield, it's rare to see a US soldier dart off all alone. On the battlefield they fight for the men they serve with; to protect each other so everyone gets home safely. They depend on each other to survive, which is why running away from battle is a soldier's last resort.
Another reason US ground forces may not be as aggressive as their terrorist opponents is technology. Groups like ISIS and Al Queda have to solely rely on surface advancement in order to win a battle. That's not that case for the United States. The US/NATO, by far, has the most advanced air and naval weaponry in the world. Typically before a major battle takes place the Air Force and Navy will conduct massive clearing out missions to significantly weaken the enemy before ground forces will even fire one round. During ground battles, air strikes are also used in order to cover their colleagues on the ground. If ground forces are getting their butts kicked, they'll draw back and let their significantly superior air power take over.
Back to the topic of the US not having to defend its own territory since WWII. The United States has carried out nothing other than offensive strategies over the past 70 years, sending US soldiers to other parts of the world. The United States is pretty uninvadable today. Not to say it's not vulnerable to terrorism, because it's one of THE most vulnerable developed nations in the world as far as terrorism is concerned. But the chances that a conventional war would be fought within US borders today is slim to none because of its extremely superior naval and air powers, let alone all of its strategic military installations all over the world that are standing armed and ready at all times. However, if a war were to be brought to American shores, the United States would confront its enemy with the most aggressive, counteroffensive strategy the modern world has ever seen.
The US military aren't full of runners. But technology and strategy has made conflicts, well, easier for the United States to win. There simply is no need to be as aggressive on the ground as the US was in WWII.