“But successful deterrence requires credibility.”
Once again you seriously underestimate Trump’s negotiating powers.
When you point out “the praise the President heaps upon Vladimir Putin, coupled with his criticisms of Germany and other European countries, has made NATO allies less confident in the United States” you forget that it has made war with Russia far less likely than it was under the Obama/Clinton watch. NATO attempts to encircle Russia with NATO allies on their border have ceased.
When you say “Within hours, North Korea threatened to attack the U.S. territory of Guam. Trump did nothing,” you miss the entire point which was to get this concession from the Chinese Regime: “If North Korea launches an attack that threatens the United States then China should stay neutral, but if the United States attacks first and tries to overthrow North Korea’s government China will stop them.”
This means Trump has the green light to retaliate if North Korea fires a missile at Guam, without China going to war with the USA. If you don’t think that is proof of Trump’s credibility, can you suggest any Obama action which got China to offer any concessions at all?