The effects and impact of the Russo-Japanese War introduced a number of characteristics that came to define 20th-century politics and warfare. Many of the innovations brought by the Industrial Revolution, such as rapid-firing artillery and machine guns, as well as more accurate rifles, were first tested on a mass scale then. Military operations on both sea and land showed that modern warfare had undergone a considerable change since the Franco-Prussian War of 1870–71. Most army commanders had previously envisioned using these weapon systems to dominate the battlefield on an operational and tactical level but, as events played out, the technological advances forever altered the conditions of war too.

For East Asia this was the first confrontation after thirty years involving two modern armed forces. The advanced weaponry led to massive casualty counts. Neither Japan nor Russia had prepared for the number of deaths that would occur in this new kind of warfare, or had the resources to compensate for such losses. This also left its impression on society at large, with the emergence of transnational and nongovernmental organizations, like the Red Cross, becoming prominent after the war. The consequent identification of common problems and challenges began the slow process that came to dominate much of the 20th century.

It has also been argued that the conflict had characteristics of what was later to be described as "total war". These included the mass mobilization of troops into battle and the need for so extensive a supply of equipment, armaments, and supplies that both domestic support and foreign aid were required. It is also argued that domestic response in Russia to the inefficiencies of the tsarist government set in motion the eventual dissolution of the Romanov dynasty.

To the Western powers, Japan's victory demonstrated the emergence of a new Asian regional power. With the Russian defeat, some scholars have argued that the war had set in motion a change in the global world order with the emergence of Japan as not only a regional power, but rather, the main Asian power. Rather more than the possibilities of diplomatic partnership were emerging, however. The United States and Australian reaction to the changed balance of power brought by the war was mixed with fears of a Yellow Peril eventually shifting from China to Japan. American figures such as W. E. B. Du Bois and Lothrop Stoddard saw the victory as a challenge to western supremacy. This was reflected in Austria, where Baron Christian von Ehrenfels interpreted the challenge in racial as well as cultural terms, arguing that "the absolute necessity of a radical sexual reform for the continued existence of the western races of men has ... been raised from the level of discussion to the level of a scientifically proven fact". To stop the Japanese "Yellow Peril" would require drastic changes to society and sexuality in the West.

Certainly the Japanese success increased self-confidence among anti-colonial nationalists in colonised Asian countries – Vietnamese, Indonesians, Indians and Filipinos – and to those in declining countries like the Ottoman Empire and Persia in immediate danger of being absorbed by the Western powers. It also encouraged the Chinese who, despite having been at war with the Japanese only a decade before, still considered Westerners the greater threat. As Sun Yat-sen commented, "We regarded that Russian defeat by Japan as the defeat of the West by the East. We regarded the Japanese victory as our own victory". Even in far-off Tibet the war was a subject of conversation when Sven Hedin visited the Panchen Lama in February 1907. While for Jawaharlal Nehru, then only an aspiring politician in British India, "Japan's victory lessened the feeling of inferiority from which most of us suffered. A great European power had been defeated, thus Asia could still defeat Europe as it had done in the past." And in the Ottoman Empire too, the Committee of Union and Progress embraced Japan as a role model.