The eroded valleys (see the post) are becoming threatening clear these weeks. A US president under internal pressure over possible links with Putin has no hesitations approaching a direct military conflict between the USA and Russia in Syria. Only to counteract the internal suspicions. The two big powers killing each others soldiers would be a revolutionary development creating a disastrous presedence. The valleys would be further eroded. The road to minor and major military global conflicts would shorten.
An erratic and/or intellectually narrow-minded US foreign policy also undermines world stability by supporting or accepting irresponsible acts and by seeming to run parallel contradictory policies. Does Washington support or condemn Qatar? A greater Israel or peace? As a prelude to new peace-talks Netanyahu, the Israeli Trump, starts a new settlement in the West Bank - for settlers having stolen Palestinian land. Will Saudi Arabia step up measures in Yemen or against Qatar, Iran or Shiites in general? More than one side can find support in Washington from a presidential statement or a US minister or other member of the administration or the Trump family.
The destabilizing can also concern countries internally. Some fractions can feel supported. Other groups like the Iranian Revolutionary Guards can increase their power because of policies opposing moderate leaders.
Of course all this is not because of the new US president. Rather, he is just one more symptom of a global development. The problem is that under responsible presidents like Obama the USA and the other big powers were stabilizing factors in the world. Of course except for localized and controlled proxy conflicts. When the one superpower become destabilized and also the relations with Russia become unstable, both will act in unpredictable ways. Yet more barriers for acts from leaders of smaller powers are diminished.
With such eroded thresholds for conflict stupid acts like the present from the North Korean regime or fractions therein can also more easily provoke countermeasures with horrible consequences. The first use of a nuclear warhead since Nagasaki would be another horrendous presedence.