What are the key economic factors that keep a country’s currency strong?

What are the key economic factors that keep a country’s currency strong? For instance; why is the usd strong when the usa has so much debt? Why is the Vietnamese dong so weak? Kuwait only has 38 billion in debt so I might understand it’s value.
In 1980-1983 IMPORT percentages were: USA 9.5% Vietnam 0%
Kuwait ranged from 34-52% and Iraq 0%
Their EXPORT percentages were: USA 9% Vietnam 0% Kuwait 55-78% and Iraq 0%
The currency was worth from 1980-83
USA I have to keep at a dollar and compare other countries to the dollar.
Vietnam 20 dong= 1usd
Kuwait 3.46
Iraq 3.22

2009-2013
IMPORTS USA 16% Vietnam 78.5%
Kuwait 26.5% Iraq 33.75
Kuwait’debt is only 38 billion I think
EXPORTS: USA 12.75% Vietnam 73.5% Kuwait 66% Iraq 41.5%

Currency Vietnam 21,000 avg Kuwait avg 3.5 and Iraq 1170 avg

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