Scholars have debated when the modern period began.
The dates when Modernism flourished are in dispute, but few scholars identify its genesis as being before 1860 and World War II is generally considered to mark an end of the movement’s height.
A smaller time frame for the era places the period between 1900 and the end of World War II.
As a cultural era, the modern era was informed by economic and social changes brought on by the industrial revolution beginning in England and Europe in the late 19th century. Coupled with the influences of the significant thinkers of the period leading up to the turn of the century (Freud, Marx, Nietzsche, Darwin), the modern era represents a period of upheaval, artistically, socially and economically.
In attempting to fix the time period of the modern era, it may be helpful to look at the artistic themes of the period.
The movement’s concerns were with the accelerating pace of society toward destruction and meaninglessness.
The combination of locomotive travel, urban expansion, factory mass-production, steam powered ships and the combustion engine lead to many changes in the way people's lives were organized. The agrarian lifestyle that had dominated Europe for its history was challenged and replaced rapidly by a new commercial model.
Though we would not want to define the modern era's time frame entirely by the advent of factory production or the automobile, looking at the era with these novelties in mind can help to understand what stood behind the modern era.
The generally agreed upon end of the modern era is the end of World War II. To punctuate this idea, some suggest that modernism ended with the dropping of nuclear bombs on Hiroshima and Nagasaki.
World War II resulted in a reformation of the political world, paved the way for the end of imperialism as it had been practiced, and established capitalism as a viable (and dominant) model of national commercial and industrial organization.