Nuclear umbrella means the a system that some countries who don't have having nuclear weapons can use as defense against the nuclear offense using the support of friendly countries. South Korea doesn't have nuclear weapons, but it is an allied nation with the USA and can get the military and nuclear support from USA.

Under the nuclear umbrella system, countries tend not to develop nuclear weapons, and it can contribute to the peace of the world. However, most countries' national defense becomes being dependent about their national defense upon the countries  who have having nuclear weapons such as the USA or China. As a result, the countrieswho have having nuclear weapons get have a bigger effect on the world. In reality, the USA manages the world securities security and, the world is governed by the USA.

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1) ALWAYS "The USA"

2) I recently mentioned relative clauses. Make sure you're paying attention to them.

3) Having is a mixed verbs. Non-continuous verbs are verbs which CANNOT be used in the continuous tense. Have sometimes cannot be used in the continuous tense because it has multiple definitions. The non-continuous definition of "have" is the possessive. So we can't normally say "countries having nuclear weapons are powerful". It should be "countries who have...'