Politics generally happen on a sliding scale, encompassing left-wing, centrist, and right-wing ideologies. Where people fall on this scale pre-determines much of how they perceive, understand, and report on world events. The political divide between individuals and organizations is the crucial backdrop to understanding the motivations for the way world events are discussed and portrayed.
Brief History of Bias in US Media
In the early decades of the US, media outlets were very outspoken about their political affiliations. People who thought a certain way knew which paper to read to have their views reinforced and opinions validated. Newspapers were free from governmental mandates about partisanship for many years, until the Fairness Doctrine was introduced as part of the 1927 Radio Act. The Fairness Doctrine required radio stations (and later TV stations) holding FCC-issued broadcast licenses to (a) devote some of their programming to controversial issues of public importance and (b) allow the airing of opposing views on those issues. This meant that programs on politics were required to include opposing opinions on the topic under discussion. Under the Fairness Doctrine broadcasters had a duty to determine the spectrum of views on a given issue and include those people best suited to representing those views in their programming. The Fairness Doctrine was eliminated in 1987 under the Reagan administration. Since then, some news outlets have become markedly more partisan.
To be clear, there are political beliefs that do not fit neatly onto a scale such as this, so please don't view this as the end-all be-all of ideologies. However, generally speaking this framework holds up in much of the world. The qualities embodied by the left side of the scale are liberalism and collectivism, with an increasing reliance on government to provide for a country's citizens as it approaches communism on the extreme left. The qualities of the right side of the scale are conservatism and individualism, with increasing reliance on corporations to provide for the needs of the citizenry.
People and/or organizations who want society to move in one direction or the other (or to stay the same!) will try to influence mass media coverage of world events in order to represent their particular ideology favorably. For instance, a libertarian think tank will try to prevent a government solution to a problem at all costs, and a liberal organization will be much more likely to focus on how a corporation is harming the environment rather than how it is lowering unemployment. These stories become the evidence that society uses to debate the best way forward (or backward!).
Steinbrecher, Anna. "A Map of Political Ideology." Politics 101: Your Basic Guide, http://politicsfordummies2012.blogspot.com/2012/09/general-political-ideology.html. Accessed 8 Jan. 2020.