Fireworks even before July Fourth. President Trump has written sharp individual letters to NATO allies in advance of next weeks alliance summit in Brussels complaining once more about their apparent unwillingness to increase defense spending as promised.

As he often does in foreign affairs, Trump framed the complaints as if those 25 countries that have yet to meet their two percent of GDP defense spending promises of 2015 are freeloaders, unfairly taking advantage of the United States. Last year only four members — the U.S., Britain, Greece and Estonia — met the two percent spending goal.

“The United States,” Trump wrote German Chancellor Angela Merkel, according to the NY Times, “continues to devote more resources to the defense of Europe when the Continent’s economy, including Germany’s, are doing well and security challenges abound. This is no longer sustainable for us.”

Most U.S. presidents urge all 28 of the other alliance members to boost spending on defense and some have. In real terms total NATO defense spending increased in 2017 for the third consecutive year.

Whether in trade issues or diplomacy Trump’s style, as you may have noticed, is more confrontational and combative than other recent presidents. That turned out well when confronting ISIS.

But the U.S. leader is alone in questioning publicly the value of the 69-year-old alliance. And in public and private Trump has spoken louder and sterner to countries and leaders who are supposed to be best friends in a dangerous world. As one result, the U.S. ambassador to Estonia is resigning in protest, as we wrote here Monday.

There is growing concern among some of them that the U.S. commander-in-chief may be planning some kind of dramatic troop reduction in response to the slow spending increases of nations that have their own economic and political problems and goals at home. Or worse, perhaps abandon or threaten to abandon the treaty’s mutual defense commitment.

The Washington Post reported Tuesday the Pentagon is studying the requirements and effects of a large-scale U.S. troop withdrawal from Germany.

This fits another aspect of Trump’s diplomatic style, unpredictability, which is supposed to give him more leverage in bargaining.

To make it even more worrisome for allies, Trump has also been quite outspoken about his desire to have better relations with Russia and Vladimir Putin. U.S. presidents regularly say that too. But Trump is underlining his unhappiness with NATO by having his first formal summit with Putin immediately after the NATO meeting.

Sowing divisions among NATO allies is a prime goal of Putin’s. Trump’s aggressive tactics fit that perfectly.

Few things would make Putin happier than to see reduced U.S. military forces based in Europe in a  strategic position to discourage or thwart Soviet, er, I mean, Russian military incursion of the invasive type Putin ordered in Georgia and annexing Crimea from Ukraine.