A guarantee of force by a different nation isn’t always in the best interest of a nation: the comparison that comes to my mind for external application of force when a canal’s “security” was threatened, is the Suez Canal.
I agree in general that it could be a bad thing. But in the case of Panama and the US I see it as a very positive thing.
For one, Panama doesn't have a military anymore since Noriega was ousted in the invasion. That's a huge blessing of stability to the country. You don't have to look far in Latin America, Asia, and Africa to see how dangerous a military is to democracy. And we don't need a military for defence, because everyone knows the US will rush to the defence of Panama if the canal were threatened. Purely from self interest, but that aligns nicely with the intersts of Panama in this case.
A guarantee of force by a different nation isn’t always in the best interest of a nation: the comparison that comes to my mind for external application of force when a canal’s “security” was threatened, is the Suez Canal.