There are numerous issues involved in this question. One of the most basic ones is the misconception that making everyone operate under the same rules will benefit everyone.
I'll make an analogy from Texas Hold-em Poker. After a long tournament, two players are left, playing head-to-head. Both operate under exactly the same rules, both have exactly the same odds to get a good hand, etc. They are "equal" in this regard.
However, one of the players has ten times more chips than the other. Thus, the shortstacked player can afford to take far fewer risks...they have to either go all-in and risk everything on a marginal hand, or try to wait for a strong hand (and risk losing most/all of their money to the blinds). And their opponent can use their power to push them off of hands that they might have had a chance of winning.
So its not, really, equal at all.
Same in regards to international economies. Fact is, building an economy involves risk, and is inevitably going to entail mistakes and failures. If you're already a developed economy, you'll be able to absorb those mistakes and failures without too much difficulty; but if your economy is weak and undeveloped, such mistakes and failures are critical, representing a significant hit to your country.
A simple example -- mining (or many other forms of natural resources). It involves a significant investment of time and money to identify a good potential mining site. Many of the places that you test will turn out to not be worthwhile (and thus, you've spent a lot of money, and gotten no return). Even when you identify a viable site, you've then got to invest even more money in building the mine.
Now, for a country like the U.S., its relatively easy to do this. But for a nation that is undeveloped, and doesn't have much money, this can be anywhere from very difficult to impossible. Thus, we get the situation we have today...where many of the natural assets of foreign countries are owned/controlled by other larger, richer nations.
Now, in some cases, this can be a boon to the country in question. It provides employment, it puts more money into the local economy, etc. In other cases, it can be a curse. People are employed at slave wages, in terribly unsafe conditions, and almost all the money goes to the foreign owners, and/or local corrupt gov't leaders.
Which direction it goes will depend largely on both the government of the country in question (Are they largely immune to corruption? Do they have strict laws to protect workers? Do they have clear policies on foreign investment that are followed strictly?), and on the foreign company that is investing there (What are the ethics of the company leaders? Do they invest their profits back into the local economy, or do they keep them for themselves? Do they apply the same standards of safety and workers' rights that they would in their own country?).
There are examples of countries that have benefitted greatly from international trade (China and India being two of the more obvious ones); and there are examples of countries that have been terribly exploited. There's no simple answer.
