I don't know enough about Philip II to answer your question regarding motivations, but I can say that the idea that Spain was capable of conquering China is absolute nonsense. It was an idea floated during the sixteenth century by overconfident colonial governors who ad a great deal of motivation to be overconfident. This was then picked up upon by the "military revolution" cohort of historians who believed that European armies were of such a qualitative difference from non-European armies that they achieved an absolute dominance in military affairs (Note: obviously not all in the "military revolution" camp).

Naturally this has provoked something of a reaction, and in the English language East Asian scholarship (Chinese language scholarship naturally being rather ahead of the curve) there has been a recent effort to show that European military dominance was really not that dominant after all. China scholars, for example, note that when the Chinese and Europeans actually did clash, it was the Chinese that won, such as the naval battles at Tunmen and Xicaowan and the various battles around Taiwan, and Japan scholars note that the Japanese actually improved upon European firearm tactics. In general there is a notion still very prevalent in the Western scholarship that China was "stagnant" and resistant to change, but this is completely false. It is partially true of the middle sixteenth century, when the Ming Dynasty was hit by a number of political crisis that left it unable to deal with the raids of the wokou ("Japanese pirates"). By the 1560s, however, the military had largely pulled its act together, reformed itself and wiped out the raiders. The Japanese got a very clear demonstration of this in the decade after the debate your article refers to, during the invasion of Korea--incidentally, the largest war of the sixteenth century. In later decades the Ming would continue to reform and experiment with its military, for example, for a brief period it adopted the European style "star fort" before it was decided to be impractical.

I'll also note that I actually listened to a interview with Hugh Thomas one time, and he is an example of the sort of scholarship that history has been desperately trying to get away with for fifty years. He actually said, for example, that the people of the Americas were better off being colonized. His last paragraph is entire sincere, he really does wish that China had been planted under the glorious white heel of the Spanish.

Unfortunately Chinese military history is highly undeveloped in the west, for a variety of reasons. Tonio Andrade (Lost Colony) is one of those fighting the good fight, as is Kenneth Swope (The Military Collapse of China’s Ming Dynasty).

A reply to /u/N3a

This is a very good set of questions and I thank you for enumerating them very clearly ;-).

Could 10,000 Spanish soldiers plus Japanese troops really conquer China?

The call to invade China was largely made by the Governor of the Philippines in Manila. I am not aware of this being taken seriously by Philip II, nor by the Council of the Indies, beyond forwarding the correspondence as was the formality.

In that period, governors, captains, chaplains alike all tended to write very daring proposals with the hope of gaining the king's ear, or if not, a suitably highly positioned patron, so that he could move up in government. The idea to invade China is not new, nor unique to one particular proposal.

But strongly speaking, in the 1570s the Spanish settlement under Legazpi and Dasmarinas was very weak and it was recognized by Philip's government as a trading settlement. Even the encomenderos were considered very poor and weak. Thus, conquest was strongly discouraged.

I tend to agree with /u/Tiako , within the confines of not getting into historical what-ifs, that to conquer China would be somewhere between very difficult and impossible. There is still ongoing debate as to how China's military and Spain's military would compare.

When the Portuguese and Spanish brought matchlock handguns and cannons to east Asia, they were copied very quickly, but even as late as the mid-1600s there was still the issue of quality, both in terms of the make of the guns, the powder, and the men using them. As late as that period, there was strong desire to obtain european guns and european gun crews, or at least trainer to lead native soldiers. This is covered in some detail in Chase's "Firearms: a global history".

Could Spain afford it?

The simple answer is no, because at the time it was obvious that the situation in Flanders would not be resolved easily in any kind of timely matter. To make the situation worse, after a long period of accommodating Elizabeth in England, relations was starting to worsen. So even in 1574, Mateo Vazquez the king's Chaplain and personal secretary wrote, "..... I can see that concern for the Netherlands preoccupies Your Majesty, and that the problems there afford little time to attend to other business ...." In particular, 1574

The 1570 was a bad time for Spain's enterprise in the Low Countries, which included various mutinies due to lack of pay, culminating in Alba's dismissal, and catastrophies at the level of the Sack of Antwerp.

Other aspects

The succession crisis in Portugal was clearly an issue that Philip resolved to handle as quickly as possible.

The Netherlands was considered very important to Philip II, and many other factors came into play. England was clearly one of them. Philip had been married to Mary Tudor under very disadvantageous terms because Charles V knew that England was key to security in the Low Countries. When Elizabeth ascended to the throne, this same concern led to accommodation. Now that Elizabeth was becoming an enemy, Spain was faced with the difficult choice of confronting her directly in England, or be forced to increase security investments everywhere in the world. Philip chose the former.

I shall stop here because as I said above, the idea of "let's conquer China with a handful of great Spaniards" is not new nor novel, so it will be good if you give a specific time frame for your questions.