Boyan Penev
Portugal, in particular, was a fairly small country. Japan was bigger, a lot more populous and a long way away. In fact, at the time, Japan’s population was comparable to that of France and possibly, depending on the source, more than that of England and France combined. Portugal had around 8–10% of Japan’s population. Think of the logistics - how would the whole thing work?
Being able to wage war and conquer a very far away region is a difficult task today, and much more difficult in the 16th century. Therefore, colonization by force can only happen when the local population is small and non-hostile, far behind technologically (say, how the Amerindians were in the Caribbean) or when they were divided and fighting between themselves and local rulers would ally with the Europeans, like in India. Japan was definitely not the first or the second, and the third, well, it was for a while, but it is questionable if any European power was able to interfere significantly during that period.

葡萄牙是一个相对较小的国家。日本更大,人口更多,而且距离很远。事实上,当时日本的人口与法国相当,可能比英格兰和法国的人口加起来还要多,而葡萄牙只有日本人口的8-10%。考虑到后勤 - 整个过程怎么运作?
能够发动战争并征服一个非常遥远的地区是一项艰巨的任务,特别是在16世纪。因此,强迫殖民只有在当地人口较少且不敌、技术落后(例如加勒比地区的美洲原住民),或当地人群体内部分裂并互相争斗,当地统治者将与欧洲人结盟时才能发生。日本显然不是第一或第二类情况,至于第三种,它一度是这样,但任何欧洲大国是否能在这段时间内对其产生重大干预还是值得质疑的。

Theoretically, a much stronger Portugal could have been able to exploit the infighting in Japan before its unification in the early 17th century, but at the time the Portuguese were much more interested in Southeast Asia and trade with China. Even then, Japan was fairly large, and the Portuguese could dedicate only so much resources to supporting their local allies. They weren’t that good at making friends either, what with the religious and cultural difference on either side (both sides considered the other uncultured barbarians with reprehensible habits and a weird religion) and the slave trade which took place initially - let’s not mince words, a lot of Japanese were purchased as slaves and sold outside of the country. Once Japan was unified, colonization was simply not feasible, and some incidents caused mostly by the Spanish led to the expulsion of almost all European traders. It is noteworthy that there was almost no reaction to it from the European countries, unlike the Opium Wars with China some 200 years later.

理论上,一个更强大的葡萄牙可能会在17世纪初日本统一之前利用其分裂。但当时葡萄牙远更感兴趣的是东南亚和与中国的贸易。即使如此,日本相当大,葡萄牙只能投入有限的资源来支持他们的本地盟友。两边在宗教和文化上的差异(双方都认为对方是野蛮人,拥有可鄙的习惯和奇怪的宗教),还有最初的奴隶贸易也让他们交朋友变得更困难 - 说白了,许多日本人被购买成为奴隶并被出售到国外。一旦日本统一,殖民就根本不现实了。一些事情造成的影响主要来自西班牙,导致几乎所有欧洲商人被驱逐出境。值得注意的是,与200年后的中国鸦片战争不同,欧洲各国几乎没有对此做出反应。

Had it been Spain - a bigger and more aggressive country - that had first found Japan, it might have been more proactive. However, I still do not think it likely that Japan or a significant part of it would have been colonized. Remember, at the same time Spain, and to a point Portugal, had problems much closer at home - revolts in the Netherlands and naval wars against the Ottomans in the Mediterranean - and colonies around the world which they also valued. Japan was way too far, way too strong, and honestly, not that big of a priority to justify the number of soldiers and the amount of money needed.
What could have happened, realistically speaking, is 1–2 autonomous ports or a small region becoming dedicated ports under essential Portuguese control (maybe with some window dressing that lets the Shogun claim nominal control). Essentially, it would be, say, Nagasaki and Tanegashima functioning sort of how Hong Kong or Macau did. But Japan becoming a Portuguese colony? It just would not happen.

如果是以西班牙 - 一个更大、更具攻击性的国家 - 发现了日本,它可能会更加积极。然而,我仍然认为日本或其中的重要部分不太可能被殖民。请记住,同时期的西班牙,到某种程度上也包括葡萄牙,在家中更接近 - 荷兰的反抗和地中海地区对骑士团的海战 - 并且在世界各地都有他们看重的殖民地。日本太远了,太强大了,说实话,并不是那么重要,无法证明需要投入大量士兵和资金。
实际情况下,可能发生的是1-2个自治港口或一个小地区成为葡萄牙控制下的专门港口(也许有一些表面上的东西让幕府声称名义上拥有控制权)。本质上,它将类似于长崎和稻叶岛,以某种方式发挥香港或澳门的功能。但日本成为葡萄牙殖民地?根本不可能。