How many more examples of presidential incompetence do we need to witness?

Here’s the latest one to emerge from the G20 summit in Hamburg, Germany: Donald J. Trump announced a joint cyber-security agreement with none other than Vladimir Putin.

That’s it: The president of the United States and the president of Russia agreed in principle to work on ways to prevent governments from hacking into others’ systems.

Oh, but wait! Twelve hours later, Trump tweeted (of course!) a change of heart; we won’t enter into that agreement with Russia after all.

Imagine that. The president backed out totally from an agreement he had his White House communications staff announce to the world.

Intelligence agencies have concluded that Russia hacked our electoral process in 2016. Then the Russian president puts this idea out in his meeting with the U.S. president — who then buys into it.

All the while, Trump keeps dissing our spooks’ expertise on this matter and then sidles up to the government believed to have sought to influence the outcome of our presidential election.

How does a White House staff at any level cope with this kind of capriciousness? How do Cabinet officers know whether to zig or zag as they try to keep the president in their sights?

Is there any wonder at all — having watched this administration stumble, bumble and fumble its way — why the president is having trouble filling so many vacancies?