Because the only winning move it not to play. This is Europe's war. Not sure why the US is involved at all. It's not like Ukraine has oil or a NATO partner.
As Viscount Cunningham famously said when he risked his fleet to evacuate troops in the Battle of Crete in 1941, 'It takes three years to build a ship; it takes three centuries to build a tradition'. Which feels like how America's new insularism is undoing all the "leader of the free world" fandom that it has carefully cultivated - and profited from - in the last 80 years.
Today the US has two strategic enemies - Russia and China - and two strategic partners, Nato in Europe and everyone in pacific except China.
The US can spend peanuts - it really isn't a lot of money in US defence terms - backing Ukraine and using Ukrainian casualties to defeat it's strategic enemy, Russia, whilst making it's other strategic enemy, China, fear it.
Or it can waver and show it's no longer the leader of the democratic world and make all it's allies in Europe and Asia not believe in it.
My big fear is that it is empowering China to dare to have it's go at Taiwan in a couple of years.
This presumes there's any amount US can spend to allow UKR to strategically defeat RU by proxy, and thereby have PRC fear it. UKR as proxy is as much limited by quality/quantity of it's human capita as it is by external support. What happens to US credibilty / desire to be US proxy in IndoPac to fight for US security interests when partners see UKR decimated to the last man despite full US assistance? The western wunderwaffles delivered to UKR have underdelivered, meanwhile US failing to guarantee red sea shipping against Houthis that US armed Saudis have failed to contain for over a decade. Single digit salvos of shit tier RU and Houthi missiles successfully penetrating Patriots in UKR and Flight2/3 DDGs in Red Sea has basically affirmed PRC the vulnerability of US hardware and validated their doctorine to deliver 1000x more fires. If anything the more US commits/show hand, and the more she reveals her (in)capability, the less her adversaries fear it. Sometimes better to commit half heartly and be thought incompetent (or indifferent) than go all in an remove all doubt. Nothing worse for US credibility than trying and failing.
Europe is probably uncomfortable/ashamed by how dependent they are on the US for maintaining the western-centric global power axis. But on the same hand are unwilling to make the sacrifices their societies would need to in order to pick up the slack. Especially now that European economies tend to be in a slump.
It certainly seems that the US is unsure whether it wants this role. The Congress is putting US credibility at huge risk right now.
Nevertheless, if the US abdicates its leadership, the free world will shrink. Even democracies have domestic enemies and all of these will be encouraged to push autocracy as an alternative to the messy parliamentary system.
