Countries have to cooperate with each other to reach a common welfare - it is better than isolation

Toefl - Countries should not isolate themselves from world but instead engage other countries.

The world has become even more connected due to globalization of people and products. Nowadays, there is no country that can provide all goods and services to its population living by itself. Thus, I believe that countries have to engage other countries to reach a common welfare, which is better than isolating themselves.

First reason is that countries are unique and each one of them has its strengths and weakness. Some are privileged by having a good climate to agriculture, others have more minerals inputs and some have more advanced technology for telecommunications, for example. These countries can trade to others what they have mastered in exchange of what they miss. Brazil for example, is a great wheat producer and sells it to China. On the other hand, China is world's biggest iron producer and sells it to Brazil. None of the countries would be able to produce the necessary amount of goods they import. The trade is beneficial for both countries.

Another reason to countries to engage is to face global issues as global warming or epidemics in a cooperative manner. To get the practical results they must act as one and work together. For example, if most part of the countries decide to implement norms to reduce the global warming, there will not be practical results if the greatest responsible for it do not agree on reducing its contribution. In addition, the others can feel compelled to not be part of this effort as well, since the biggest contributor will not cooperate. To get results on global issues they must act together.

For these reasons I believe that countries instead of isolating themselves must do efforts to work together. They can help each other exporting products they have in excess and importing the ones they lack the knowledge or the inputs to manufacture. Also, when they embrace global issues unified they have better outcomes than if they were working by themselves.

... greatest responsible for it do[does] not agree on ...

you should use does not do because (I, YOU, THEY, WE = do) (SHE, HE, IT = does)