Why did Democrats get so upset about a 4th of July military parade?

Because they hate the military and America. Pretty much every Democrat politician blames the US for the problems of the world and thinks our soldiers are stupid baby murdering thugs.

According to Democrat politicians the only reason that radical Muslims are waging a war of terror against everyone else on the planet is because the US doesn't support the mass slaughter of Israelis.

According to Democrat politicians Iran is supporting terrorism and developing the Atom Bomb because the US didn't show Iran enough respect.

But according to Democrat politicians Russia's invasion of the Ukraine wasn't due to Obama being soft on Russia.

Democrat politicians view our soldiers as mass murdering monsters.  We know better. We know that our brave soldiers have increased the chance that they will be killed in order to reduce the chance that they will accidentally kill civilians. But to Democrat politicians, starting with Kerry, our soldiers go out of their way to kill babies. Of course the Democrat politicians hate those baby murdering soldiers. Which is odd given that the Democrat politicians have all come out in favor of killing born alive babies recently.

Trump gave a completely patriotic speech that extolled America-- and in which "I", "My", etc didn't appear a million times--and the left worried about our military running over civilians with the tanks that paraded as part of the same event. What sort of liar or lunatic must one be to think that the same soldiers who risk their lives to minimize the number of Syrian Muslim civilians they kill would run their tanks over their fellow Americans?

As recently as the 1960s military parades were common. Not to glorify war but to acknowledge that our freedom has been won with the blood of Americans and if we ever stop being willing to shed that blood then we will lose our freedom to the new barbarians--whomever they might turn out to be.

Leftists are rarely people who actually are willing to fight or even lose money defending our freedoms and they are incapable of identifying with those who are. While most Americans haven't been called on to risk their lives for their country most of us can identify with and recognize the special value of those who do.

A logistics officer who served in the US Army in Germany during the 1980's had little chance of dying for his country but his contribution to America, deterring the Soviet Union, was far greater than the contribution of some Ivory Tower academic who studied gender theory or who deconstructed Milton.

The fields the left excels in do little to help Americans in general. There is no basis for the sense of superiority that the left feels over our soldiers. Soldiers are a lot smarter than the average leftist.

For Democrat politicians and their left wing base the highest human value is intellect. For Jesus and the vast majority of humanity it's how willing a person is to truly love their neighbors when doing so involves more than a gesture.

That's why conservatives, and even more so conservatives who are religious, who give the most of their own money and time to help those in need.  Leftists wear symbolic wristbands while conservatives wear sweatbands when the invest their own sweat equity to help the poor.

Democrat politicians and the left hated Trump's celebration because it celebrated America and that's something they can't stand.

Until America has been remade as the left wants it to be--eliminating the Constitution and making politicians our rulers not our representatives--the left will condemn any celebration of our freedom and our willingness to defend that freedom.