Note that Europe and Japan were rebuilt with US' capital and machinery after WWII. Also a key difference (to a typical developing country) is that both of them have already industrialized before being completed destroyed in WWII so they have established institution and industrious workforce, which can be regarded as "internal resources". It will take them much much longer to re-establish their industry if US did not pour capital and machinery onto them. Most of the developing countries will not be so "lucky" because they are not in their camp.
I do agree that the developing countries need to work really hard to "strengthen" themselves economically though. Economic development works with a combination of capital, technology, workforce, and resources, of which developing countries find themselves lack and are difficult to improve.
