Final answer:
The Mongols' failure to expand beyond China was due to their inexperience in naval warfare, internal fragmentation and infighting, resentment among Chinese subjects, and unsuccessful overseas military campaigns.
Step-by-step explanation:
The Mongols faced significant challenges in their attempts to expand beyond China, primarily due to the strengths and strategies of the neighboring peoples and natural obstacles. Although the Mongol Empire under Genghis Khan was adept at land conquests, they were relatively inexperienced in naval warfare. This proved to be a critical deficit in their campaign against the Song dynasty situated in southern China, which had a landscape crisscrossed with expansive river systems. Furthermore, the Song's economic might, despite their military weaknesses, and the presence of formidable fortresses in the region posed challenges to the Mongols.
Fragmentation of the Mongol Empire also played a pivotal role in their inability to expand. Infighting among Genghis Khan's descendants led to civil war and subsequently, to the division of the empire into separate realms. This internal disunity weakened Mongol cohesion and undermined their military capabilities, making further expansion difficult.
Additonally, the Mongol taxation system and expropriation practices alienated many Chinese peasants, creating resentment and resistance. Coupled with cultural differences and the impression of foreign rule being an affront to local sensibilities, the Mongol Empire's grip on its subjects loosened overtime. This internal discontent made it difficult to maintain control, let alone expand further.
Lastly, despite military prowess, the Mongols were unsuccessful in their overseas ventures such as the attempted invasions of Japan, which failed due to logistical issues and natural disasters like the famous 'kamikaze' typhoons. Their expansion into Southeast Asia also faltered, lacking the naval skills required for effective long-distance sea invasions.