In terms of quantity and variety, the United States has contributed most of the military material Ukraine has been receiving. This is partially by design, since the United States has, since the 1949s, been the largest producer of weapons and munitions in the West. After NATO was created in the 1950s, the dominance of the U.S. in this area continued, even with adoption of NATO standards which the Americans and Europeans jointly decided on and observed. This led to NATO members having a wider variety of weapons to choose from. NATO nations also established a war reserve of sorts and a common policy on keeping retired weapons in storage for a while until they were considered truly obsolete and not just the “previous model”. All this came in handy when Ukraine was invaded by Russia in February 2022 to, among other things, prevent Ukraine from joining NATO. Ukrainian forces still use a lot of the same weapons the Russians use (both were part of the Soviet Union) but as those weapons are lost in combat or wear out, Ukraine replaces them with NATO standard equipment.
This has put a lot of pressure on the United States to revise its production schedule for the most heavily used weapons and munitions in Ukraine. The U.S. has contributed at least $12 billion worth of the weapons and munitions of Ukraine. To do this it was necessary to deplete stockpiles the American forces depended on. This is particularly true with missiles and artillery ammunition as well as artillery and HIMARS launcher vehicles for GMLRS guided missiles. GMLRS was successful enough in Iraq, Afghanistan and elsewhere after its introduction in 2005 that the United States built up a large stockpile of GMLRS missiles and spent less on 155mm artillery ammunition for towed and self-propelled artillery. Guided weapons were more efficient, but artillery was not as obsolete as expected. For the last ten years production of most of these munitions was at peace time levels, sufficient to satisfy training needs and some combat.
Since Ukraine was not yet a NATO member, Russia was not at war with NATO and NATO and Russia preferred to keep it that way. It was up to over half a million Ukrainian troops, most of them recent volunteers to do the fighting. That was made possible by a rapid and sustained supply of weapons and services from NATO nations. This meant mainly the United States. Now the American military leaders have to convince the legislature to provide the money needed to replenish the stockpiles of weapons and munitions. To increase production sufficiently to replace what was sent to Ukraine requires providing a multi-year contract to cover the time it takes to increase annual production. You don’t need a permanent annual production because no war lasts forever and no nation can afford to keep building unused munitions, especially the expensive “smart” guided shells and rockets. The same rule applies to weapons like cannon-artillery and HIMARS rocket launchers. The justification of this spending is easy to make for the United States. Russia has provided a current and ongoing military threat with Russia. No one ever expected another Hitler to show up in Vladimir Putin who is determined to restore the Russian empire no matter what the cost. Then there is China, which is more of a threat but not as self-destructive as Russia. China is even trying to persuade its military partner Russia to back off on its self-destructive expansion efforts.
Meanwhile Ukrainian forces could use more HIMARS launchers because currently there are more GMLRS missiles available than launchers to use them. When the Ukraine War began, there were about 30,000 GMLRS missiles available, out of the 55,000 produced since the weapon was developed two decades ago. Production continued, mainly to replace missiles used for training and for export to new users. That kept the cost of each GMLRS missile under $200,000 each for a combat-proven and very effective weapon. Ukraine demonstrated that such a weapon would be needed more than anticipated in a major war, rather than in the battles against scattered irregulars (Islamic terrorists). A near-peer force like Russia has more useful targets to go after like supply storage and distribution sites, headquarters and all manner of support facilities. The Ukrainians are demonstrating how this and how it saves lives of the force using GMLRS. The lesson here is each nation using GMLRS needs to maintain a larger stockpile of missiles and launchers in peacetime. This also applies to a lot (but not all) of the weapons and munitions supplied to Ukraine.
Ukraine could develop improvised launcher vehicles in cooperation with the U.S. and some other NATO nations that considered such a move but found it was cheaper to buy the HIMARS vehicle. HIMARS consists of three basic elements; a heavy truck plus missile storage/launch containers which contain some electronics but are basically to store and launch the missile. Finally, there is the fire-control system needed to put the target location in each GMLRS missile and communications equipment to receive that information in encrypted form. Poland was particularly eager to design and build its own GMLRS launcher but was prevented from doing so because it was faster and cheaper to buy the HIMARS vehicle. Such expedient and more expensive improvisation is justified and cheaper in wartime and for Ukraine and nearby nations like Poland, Finland, the Baltic States and Romania because the fight to stop Russia provides more motivation and the opportunity to quickly test (verify or modify) improvisations to get them into use to win and halt the fighting.
NATO thought it was out of work after the 1991 dissolution of the Soviet Union. What prevented disbandment was the demand from the newly liberated (from communist governments and Russian domination) Eastern European nations who now wanted to join NATO. The East Europeans made a convincing case that Russian could once more become a threat. This was less convincing to the U.S. and the founding NATO members in Europe but the memories of what happened in Germany between the two World Wars still has some impact on West Europe and a lot more on East Europe. The eastern NATO members were right.
During the 1990s, before the East European nation convinced their Western brethren that there was still a threat from Russia, NATO had agreed to remain in existence to see what it could do to fill the need for peacekeepers worldwide. This did not need the large military forces Cold War operations demanded so NATO nations eagerly reduced their armed forces. In East Europe the less affluent new NATO members were spending more of their annual GDP replacing Russian weapons with more effective Western models. All NATO nations were alarmed at the growing economic/military threat from China, but for Eastern Europe Russia was a more immediate (physically) threat. It was a strange replay of World War II with China replacing economic superpower Germany and Russia behaving like aggressive Nazis. In both cases, the key to victory was the ability to outproduce the opposition. China is producing more weapons than Russia but faces domestic political problems from a newly affluent population which opposes a major war that would definitely cripple the economy without any real benefit to most Chinese.