The transition from a world characterized by 'frontiers' to one dominated by 'boundaries' is a profound geopolitical and historical shift. A frontier typically refers to a zone of transition, often sparsely populated, undefined, and marking the edge of settled or controlled territory, offering opportunities for expansion and exploration. A boundary, in contrast, is a precisely defined, often linear, and internationally recognized line separating two political entities, signifying the limits of sovereignty. The modern world has largely seen the disappearance of frontiers and their replacement by clearly demarcated boundaries due to several interconnected reasons:

1. Global Exploration and Mapping: By the late 19th and early 20th centuries, most of the Earth's landmass had been explored and mapped. The age of discovery, which saw European powers venturing into unknown territories, gradually concluded. This comprehensive geographical knowledge eliminated the vast 'terra incognita' that once constituted frontiers.

2. Colonialism and Imperial Expansion: European colonial powers systematically carved up the world, particularly Africa and parts of Asia, into defined territories. The 'Scramble for Africa' in the late 19th century is a prime example where arbitrary lines were drawn on maps, converting vast frontier zones into colonial boundaries. These colonial boundaries often persisted even after decolonization, becoming the international borders of newly independent states.

3. Rise of the Nation-State System: The Westphalian system, which solidified the concept of sovereign nation-states, emphasized territorial integrity and defined borders. For a state to assert its sovereignty effectively, it needed to know the precise extent of its territory. This led to a global push for demarcation and recognition of international boundaries.

4. Technological Advancements:

   • Transportation: Improved transportation (railways, steamships, later automobiles and aircraft) allowed for easier access to previously remote and inaccessible areas, facilitating their settlement, resource exploitation, and integration into national territories.
   • Communication: Telegraph, radio, and later satellite communication enabled central governments to exert control and administer distant regions more effectively, turning frontiers into manageable administrative units.
   • Surveying and Cartography: Advances in surveying techniques and cartography allowed for highly accurate mapping and demarcation of borders, replacing vague frontier zones with precise lines.

5. Population Growth and Resource Scarcity: As global population grew, pressure on land and resources intensified. Previously undesirable or remote frontier areas became valuable for agriculture, mining, logging, or settlement. This increased demand necessitated clear ownership and control, leading to the establishment of boundaries.

6. Economic Development and Resource Exploitation: The industrial revolution and subsequent economic development created an insatiable demand for raw materials. This drove nations to explore, claim, and exploit resources in previously undeveloped frontier regions, leading to their integration into global economic systems and the establishment of administrative and political boundaries.

7. International Law and Diplomacy: The development of international law and institutions (like the League of Nations and later the United Nations) promoted the principle of territorial integrity and the peaceful resolution of border disputes. This encouraged states to formally define and recognize their boundaries through treaties and agreements, further solidifying the global network of borders.

8. Security Concerns: Clearly defined boundaries are essential for national security, allowing states to control movement across their borders, manage immigration, prevent smuggling, and defend against external threats. The ambiguity of frontiers often led to conflicts, making clear boundaries a preferred solution for maintaining peace and order.

In conclusion, the replacement of frontiers by boundaries is a testament to the completion of global exploration, the consolidation of the nation-state system, the impact of colonialism, technological progress, and the increasing pressure on land and resources. While some 'internal frontiers' might still exist in terms of undeveloped or sparsely populated regions within national borders, the concept of vast, unclaimed, or undefined international frontier zones has largely vanished from the modern geopolitical landscape.