If you’re wondering why it matters that President Donald Trump withheld military aid to Ukraine while he was requesting political favors from its new president, think about the Ukrainian soldiers fighting a nasty proxy war against Russian-backed separatists.

America is Ukraine’s ally in this fight. Ukrainian commanders, battling to hold their country together against a five-year onslaught by Russia, have been depending on U.S promises of military assistance. In life or death situations like this, America’s word is its bond. But suddenly, in mid-July, American commitments seemed to Ukrainians to have become Trump’s political tool.

Why is this more than just another Trump vs. Democrats mud fight? Because the Ukraine issue is about compromising U.S. national security — and direct pledges to allies — for the president’s personal political gain. That’s what’s so outrageous about Trump’s alleged push to get dirt on his potential 2020 rival, former Vice President Joe Biden, in a July 25 phone call with newly elected Ukrainian President Volodymyr Zelensky. Not for the first time, Trump put himself above his country.

Trump isn’t denying the basics. He confirmed Tuesday he had held up delivery of a promised $391 million in military aid for the Ukrainians in mid-July, before his call to Zelensky. Trump claimed he wanted to pressure “Europe and other nations to contribute to Ukraine.” Trump had suggested Sunday that in the July call he had urged Zelensky to investigate Biden’s son’s work for a Ukrainian gas company.

Forget the political jousting between Trump and Biden and consider the Ukrainian soldier in the field fighting to save his country. He has a nightmare communications problem because he can’t talk reliably with his commanders. Russia has been hacking or jamming Ukrainian military communications since it seized Crimea and began supporting the separatists in 2014.

America wanted to help fix this battlefield communications disaster. One item in the $391 million package Congress appropriated is a secure system made by L3 Technologies, a unit of Harris Corp., that could allow the Ukrainians to maintain contact despite Russian interference. The L3 equipment was ready for delivery in July when the company was told no, there was a hold, the equipment couldn’t be shipped, according to a congressional source.

L3 and other companies supplying Ukraine contacted leading Republican members of Congress, such as Sen. Lindsey Graham, R-S.C., to find out why there was a delay. The members of Congress learned that the Office of Management and Budget had stopped the shipments, on orders of the White House, to conduct “due diligence” about corruption and the newly elected Zelensky, according to one knowledgeable source.

Graham warned the White House: “You can’t send a signal that we’re going to back out of the deal,” the source said, adding, “It’s one thing to do ‘due diligence’ and another that we’re changing our posture.” These national-security arguments eventually prevailed, and the Ukraine assistance was finally released Sept. 11.

People who regard this latest Trump affair as just more political noise should examine an op-ed published Tuesday by seven freshman Democrats who served in Iraq, Afghanistan or other military deployments. These aren’t reflexive Trump bashers. They’re mostly from swing districts where impeachment isn’t popular, but the Ukraine case pushed them to recommend that step.

Here’s what Houlahan and the six other national-security veterans wrote in the Washington Post: Trump “allegedly sought to use the very security assistance dollars appropriated by Congress to create stability in the world, to help root out corruption and to protect our national security interests, for his own personal gain.”

The former military and intelligence officers saw Trump’s actions as a potential violation of his oath of office, requiring urgent investigation, and they’re right. This isn’t just another partisan fight. It goes to the essential obligations of a commander in chief.