This is my theory, so TIFWIW...
I think there are three angles at play for the US. Russian one, the European one, and the Asian one. All three rest on Ukraine.
The Russian angle is that by monopolizing gas they can largely neutralize Europe from supporting the US, dismantle NATO and extend their borders up through Ukraine and make it more defensible, Russia may not even need to invade as long as the saber rattle loud enough to force some backroom hush hush deals with Germany. That’ll open up Nordstream 2, remove Europe from the equation, and force concessions from the US. The worst part from our standpoint would be a resurgent Russia on a world stage as a much larger competitor that we had spent the prior century dismantling.
The European angle is such that European foreign policy is largely controlled by the US today. European foreign policy is primarily concerned with Eastern Europe, and a NATO presence there plus a puppet state in Ukraine keeps the Europeans tied to the US. If NATO is pushed out of Eastern Europe, and Germany accepts Russian gas, the threat of Russian gas cutoff outweighs the red boogie man that NATO was designed to stop. Better stated, Germany would face a bigger strategic problem that wouldn’t require the US for a solution. With a Russian Ukraine and a removal of US influence from Eastern Europe the Germans and thereby Europeans are less likely to need the US.
The Asian angle is we’ve been trying to pivot to Asia for almost a decade. We’ve made the calculation that we need Europe economically and the five eyes plus France militarily for the region. We are willing to piss off the Russians, even threaten war, to keep European dependence and make it so they have no choice but to follow our lead. If Ukraine falls to Russia US leverage goes poof, and the Germans can choose to back us, or open the door to the Chinese. We aren’t confident “freedom and liberty” is enough of an argument to keep them actively involved in our side.
One last piece is the currency argument and US leveraging. If the faith in the dollar is predominantly based in US strength. Any fracture along the way, the decline of NATO or the loss of Taiwan, could seriously damage the dollar and make our lives more difficult.
I do not believe the military is the end all be all of the dollar, and we have turned Saudi Arabia and their oil into a puppet state effectively. Another huge plus is our massive advanced economy and stability surrounded by two oceans and no real threats will always make the US one of the powerful economic engines on the planet.
Our debt is equivalent to someone borrowing $60k on a $50k salary, and paying $93 a month. We could conceivably keep borrowing forever as long as interest stays low. Planet instability has made every additional dollar borrowed even cheaper.
If a military blow were to happen and the world order post WW2 is no longer sustainable, all a very real possibility, the world would become more unstable. International currency would still need a reserve, and the only two viable options would be the Euro or Dollar. Most likely case would be a greater mix of Euros in the reserve portfolios for most countries. Our $93 monthly payment could jump to a little over a $100.
And if it’s more expensive to borrow, it may force a higher degree of competitiveness for us here.
Just my three cents.