What did President Eisenhower mean when he compared Vietnam to a domino?The Cold War

America was paranoid about the communist take over in
South East Asia. President Eisenhower came up with the domino theory which
was

you have a
row of dominos set up, you knock over the first one and what will happen to the last one
is a certainty, that it will go over very
quickly.

Which meant that if
one country fell into the communist sphere of influence the rest would follow like a row
of dominos (falling)

What Eisenhower was trying to say that
if they allowed communist takeover in Vietnam it would lead to the takeover of Laos,
Comodia, Thailand, Burma and maybe India. 

Which sums up
the entire south east Asia. south east Asia was very important to the US for its
military and trade.

hope this
helped.