a bridge too far

if country E and country F are enemies 

and a bridge is built between country E and country G 

then country F could move military troops and military vehicles 

over the bridge through country G to country E 

in order for country F to invade country E 

but then country E would destroy part of that bridge 

so that the bridge became unusable 

until the bridge is repaired after the war 

the wise thing would be to move military troops and military vehicles 

by ship or aircraft 

so the main purpose of the bridge 

is civilian commercial transport of products and workers 

in order to strengthen the economies of country E and country G 

during peacetime and possibly during wartime