I hope that most Americans are aware of the danger below.

I understand why many Americans sympathize with the slogan "America first". Indeed, America has been working for the benefit of others for too long and too much. Free trade, which the US had coveted so much, was used by Germans, Japanese and later Chinese. America has accepted a huge number of immigrants, although they were needed, providing them with better life. And I admire how the minorities in the US have achieved far better status in comparison with 1960 s.

It is good that the US demands more efforts from its allies in their own security.

But---things should be well balanced. If "America first" turns into "we Americans do not care about such and such" attitude, then US will be very quickly forgotten, the power vacuum will be usurped by regional powers.

This will be a situation "America who?" where not only US interests will be neglected, but also even the norms stipulated in the UN Charter will be disregarded.
A balanced attitude would be needed.