Only reason that USA is proclaiming that "we will nuke any muthafucka who lays even a finger on Japan" is because Japan by itself is fully capable of defending itself from any invasion, including one by Chinese Navy.
They would never proclaim such for other countries like Philippines or Taiwan ,because they would have to make good of their claims or lose face.
In as less than 1 month, we can create a thermonuclear bomb. We have enough military grade plutonium and uranium for over 1000 warheads. America's promise of nuclear umbrella is but a pragmatic way of preventing Japanese nuclear ambitions to be realized fully. As we speak, Japan is reclaiming and restoring its natural ambition and power projection capability. A nuance from the policy of some 2-3 decades prior.
To quote Jon Snow, 'Winter is coming...'
Is Japan Developing a Nuclear Weapons Program? | Global Research - Centre for Research on Globalization