Which ones are of strategic importance to the US

None of them. The americans are in Afghanistan because of Alkadia. When you attacl the US, the americans take it personally:o
The Americans are like juveniles ... they just loved to fight. They are/were involved in all of the above states at various times or others either
directly or through proxies. Sometimes for no other reason except to make life difficult for their arch rivals (such as Russia, China, Libya etc).
In most cases, their intervention were for the country's rich resources and/or strategic importance. In other cases, to defend the US petro-dollars
by reminding key producers to toll the line.
After "Black Hawk Down" incident in Somalia, the US increasingly focused on leveraging UN Command to do the overt operations and CIA to do
the covert assassinations.
They were actively engaged in Africa through the 1990s in competition with the Soviet Unions. However, with USSR disintegration, the US shifted
their focus to the Gulf and Eastern Europe ceasing Africa to The Peoples' Republic of China.
Now they are in a military (Arab Spring in N Africa) and diplomatic blitz through Hilary Clinton to try and win back these resource-rich nations from China.
Another key point to note is that the US is again realigning their strategic interests around the world with focus on Asia. Europe has been
enjoying a low investment and expenditure in defense, but this is about to be changed!
Key point to note is:-
Do bet on the USA to help ... they wouldn't be around if you need them!