Many years ago, I was involved in Libertarian Party politics. But after numerous arguments on foreign policy, I became frustrated and left. Isolationism, and attacks on “neo-cons” and claims the US should not be the “world’s policeman” rang hollow to me and lacking in substance.
In the Founding Generation of the United States, the country was quite weak and did not want to enrage any European power or the politics of Europe. In fact, the United States did not have much of a navy until President Thomas Jefferson raised funds to fight the Barbery Wars in northern Africa. However, as time passed generation after generation, the US became more prosperous and grew larger. With such growth, the footprint of the US became greater. This meant, no matter what the US did on the world stage, someone somewhere would take issue with the US.
When arguing with an Isolationist, they assume all countries are of the same size, power, and influence, or at least to some degree. This is just flat-out false. Here are some images showing how the US ranks. First, population:
(From https://www.worldometers.info/world-population/population-by-country/)
Manpower is manpower feeding military might. Unleashing people to prosper, however, is a different matter. Here are countries by Gross Domestic Product:
(From https://www.worldometers.info/gdp/gdp-by-country/)
As seen above, the United States is substantially wealthy. The US has an impact worldwide merely in private donations to charities helping people. Yet, with wealth, the US has a sizable military. Thus this image:
(From https://www.statista.com/statistics/262742/countries-with-the-highest-military-spending/)
So what of Russia and Ukraine? The standard thought on war is one side wins and the other side loses, but this war is about attrition aimed at running Russia down. Germany, Poland, Finland and Baltic countries have been run over by Russia before and will not sit back and let Russia take parts of Ukraine. Russia has lots of territory to defend and by weakening Russia, other countries will fill the vacuum. By making Russia expend a great amount of effort to take Ukraine, Russia becomes weaker over the long term.
(From https://www.statista.com/statistics/262955/largest-countries-in-the-world/)
The result frees other countries in the region to take action in their own interest, but not always for good. This is why Azerbaijan is eating up Armenia (https://www.cfr.org/global-conflict-tracker/conflict/nagorno-karabakh-conflict). Ukraine, in the end, may be able to sue for peace or perhaps Russia will retreat, but a mixture of the two is more likely where Russia pays Ukraine for land it wants.
Mostly you're right that wars weaken the combatants fighting them, but because of the war in Ukraine, Russia's ties to China, India, Saudi Arabia, and a number of other countries have been strengthened.
They are now in a much better position militarily and financially than they were before they went to war in Ukraine--and we set them up for those alliances when we insisted on stepping in and fighting a proxy war on Ukrainian soil after provoking the Russians by building biolabs (some of which appear to have been experimenting with highly dangerous pathogens) and egging on the Nazi Azov Battalion in their abuse and mistreatment of Russian-speaking Ukrainians. Also--the CIA sponsored a color revolution to install a regime that was hostile to Russian interests and eventually put Zelenskiy in power.
Our allies around the globe watched these events with concern, if not outright alarm because they didn't want to be next in the CIA color revolution sweepstakes. This has had serious consequences for us in the world of diplomacy.
If we hadn't interfered, Russia might still have taken over Ukraine (but they wouldn't have had nearly as tempting an invitation to do it), and their GDP combined wouldn't have significantly changed Russia's GDP or economic ranking--and we wouldn't now be in a position where the value of the American dollar is badly eroding on the international stage while Russia and their allies are switching to a gold standard to stabilize their currencies. China is still pretty wobbly financially as of yet and may yet descend into real economic depression, but--the Saudis and others might just be sufficiently motivated to bail them out if they do. It all depends on how annoyed they are by Biden when that necessity arises.
Russia also has access to as much natural gas and oil as they need and they have allies willing to buy their surplus while we're dealing with a headcase in the White House who's shut down major gas and oil fields, starving us of fuel while begging OPEC to sell us more--but only until he tries to force us all to convert to electric vehicles and ditch gas stoves, heaters, and furnaces. This is a recipe for disaster.
The war we should have been far more concerned about all along isn't the war on Ukraine, but the Democrats' war on the rest of us.