Sagot :
The event that led to an heightened focus on national security by the USA was the September 11 aka the 9/11 bombing of the twin tower by terrorists.
What is National Security?
Generally, National security is simply defined as the capacity of a nation to activate its state military in order to assure its borders are safe from intruders and also successfully defend against international threats.
In conclusion, This may include military attacks and also attacks on terrorists. Hence after the September 11 aka the 9/11 bombing of the twin tower the USA heightened its focus on national security.
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