Monguls did not have the need to have a navy untill late into their conquest because They were in Asia, and they could easily expand west so they did not need a real navy.
When the Monguls wanted to take over Japan however, A Mongul navy was built which was one of the most advance and effective, with experienced Chinise Sailors used. The fact Japan wasnt conquered, is because the 1st expedition was destroyed by Tsunami weather, which you cant really do anything about.. The second expedition goto the islands surronding Japan, but the men were plauged with Diease outbreaks ect and had to retreat to Mainland China or die painfully
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If i recall didnt the Monguls invade China partly by using naval boats in the rivers, or was that in one of China's civil wars
One of the terms of being a Superpower, is power projection.. Now China has ICBMs and further Missiles, but that is not real power projection, For China to become a Superpower it needs power projection and that power projection is a navy, and Aircraft Carriers.. Look at the British Empire.. its Army was not really good, the Red Coats got whooped by Afganistan backed with Russian aid in the time of the great game. But yet Britain has had a empire which the sun never sets, and which has controlled 1/4 of the world! This is all thanks to The Royal Navy really, and power projection.
Ultimately sure you dont HAVE to have a Navy in the real world.. but thats if your content with being a middle/great power and not a superpower, because unless you have a navy your power projection is limited.
China's Military maintask right now is converting from a brown water navy, which is limited to coast defence, and partial power projection to islands like Taiwan and Japan, To a blue water navy which is like The United Kingdoms Navy, United States of America, Russia, Japan and India