America's European allies may be starting to feel this way about the United States.

Donald Trump has shown a special knack for starting diplomatic spats with longtime US allies — the United Kingdom, Australia, Canada, and Mexico all found themselves on the president's erratic firing line within his first couple of months in office.

Germany is the new enemy, after a recent visit to Europe left leaders there despondent about US relations and prompted Chancellor Angela Merkel to make the unprecedented statement that Europe could no longer fully count on the US as an ally.

Rather than letting things cool, Trump ratcheted up tensions upon his return with a tweet that not only constitutes shoddy diplomacy but belies a lack of understanding of both economics and geography:

There are several problems with Trump's focus on the trade deficit with Germany. First, bilateral trade deficits are not particularly useful indicators of countries' economic performance in relation to others. More importantly, Germany is part of the European Union and therefore trades with the US and other nations as a bloc. That makes its bilateral trade deficit with the US even less relevant.

Germany is also a member of the eurozone, which means the European Central Bank, not Germany's Bundesbank, sets monetary policy, undermining claims by members of the Trump team that Germany was somehow manipulating its currency to maintain a competitive exporting edge.

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The thing is that there is a widespread complaint about Germany shared by not only the Obama administration but also many European states: The country saves too much and spends too little, creating internal economic imbalances that make it harder for struggling countries elsewhere in Europe to compete. Trump could have seized on that to make a case for a more positive German contribution to the world economy.

But that's far too nuanced a point for this American president, whose visit to Europe was marked by all kinds of diplomatic strife, from ridiculous handshakes to unceremonious shoves.

Germany bad, America good. That's best Trump could do. No wonder Merkel was left with such little confidence. When it comes to leading the free world, she knows it's up to her now.