The US has also been challenged by the increasing importance of China in recent years.
Although China is not a match militarily to the US, it is an economic superpower. It is expected that China's economy will be larger than the USA by around 2020. In 2016, Beijing overtook New York as the 'billionaire capital' of the world.
China is increasingly developing new industries and technologies. It has also entered the space race.
Some in the US feel that global trade is unfair, that China has taken advantage of American technology to increase innovation and trade, and that American companies and jobs are struggling as a result of cheap Chinese labour and products.
In early 2018, President Trump announced plans to introduce trade tariffs of 25% on a range of Chinese products, including steel and aluminium. In response, China has announced new tariffs on US pork, wine and other food products.
China is also using its new found wealth to increase its influence in Asia and particularly in Africa. By way of response, the USA has increased aid and trade with many African countries to ensure US influence is maintained within the continent.
Despite their differences, the USA and China depend on each other economically. Both countries are major sources of investment and export markets for each other.
The US is seeking more co-operation with China over the problem of North Korea pursuing nuclear weapons.