Does Trump's motto "America first" mean "UN second" for the United Nations? When he was still a presidential candidate, Trump had frequently belittled the UN. And some in the White House believe the world body acts as a global bureaucracy that infringes on the sovereignty of individual countries. In brief remarks to the UN on Monday, Trump chastised the world body's bloated bureaucracy and budget, saying, "we are not seeing the results in line with this investment."

Although he pledged to the UN that the United States would be "partners in your work" to make the organization a more effective force for world peace, Trump didn't strike conciliatory tones at his first speech in front the United Nations' general assembly on Tuesday.

According to the United Nations Association of Germany, the 2015 UN budget totaled at least $40 billion (33.48 billion euros), about as much as German firm ThyssenKrupp's annual revenue. But unlike the steel producer, the UN lives off its 193 member states' dues.

A 40-billion-dollar budget

Three main pillars constitute the UN budget. Regular dues, determined at regular intervals through an allocation ratio, make up the first pillar. According to this ratio, richer countries pay more and poorer ones pay less. Not only is the US home to the UN headquarters, it also accounts for around 20 percent of the regular UN budget. In 2016 and 2017, the US's regular dues amounted to $5.6 billion, with the lion's share of the money being put towards key bodies like the General Assembly, technical committees and the UN Secretariat.

Member states' financial support of peacekeeping missions, which is substantially larger than their regular dues, is the second pillar of the UN budget. Around 100,000 people are currently on duty on behalf of the UN in 16 countries. The last General Assembly in July earmarked roughly $7.3 billion in order to sustain this effort until mid-2018. Costs are distributed analogously to the allocation ratio.

However, permanent members of the Security Council also bear the costs for the share of the poorest countries, along motto: more power, higher dues. That's how the US share adds up to almost one-third of all costs of peacekeeping missions.

Complicated housekeeping

The UN budget's third pillar is member states' voluntary contributions. This bucket completely or at least partially funds programs and fonds of the United Nations. Examples are the development program (UNDP), the children's fund (UNICEF), the population fund (UNFPA), the World Food Programme (WFP) and the UN's refugee agency UNHCR.

These programs to a large degree depend on member states' voluntary contributions. In the process, states can set their political agenda through their financial support. Here, too, the US for years has been the biggest backer of most programs. Of the UN's overall $40-billion budget, voluntary contributions ($25 billion) make up a much bigger piece of the pie than compulsory ones ($15 billion).

Trump's biggest lever

Earlier this year, Trump's ambassador to the UN, Nikki Haley, already showed in which direction the US president is headed politically: At the beginning of 2017, the UN had reduced its peacekeeping budget by roughly $600 million. 

Haley said the cuts "were only the beginning," and wasn't willing to ascertain whether the US was willing to continue paying its dues to the same extent as before.

The greatest lever the US has in terms of cutting costs is reducing voluntary contributions. In April, the US government said it would let its contributions for the UN population fund (UNFPA) run out. Yet other member states are much bigger donors, at least when it comes to UNFPA. What would really hurt the UN is cutting the UNHCR and the environment program (UNEP) budgets.

Moreover, experts suspect that Trump wants to make further cuts to peacekeeping missions, for which the world's largest economy currently provides almost 28 percent of the budget. It's quite possible that Washington will limit this share to 25 or even 20 percent, as it had called for in the past.