This is more of a linguistics issue than a history issue but I can probably help out a bit:

Firstly, the Japanese language has three "alphabets." The first is Kanji, which are the Chinese characters. They function just like the Chinese characters in, well, Chinese, and are a direct result of Prince Shoutoku importing Chinese ideas and philosophy (as well as Buddhism) in the Nara period in the 600s.

Then, you have hiragana. This is the native Japanese alphabet. It is typically used for most Japanese phrases.

Finally, you have katakana. This system is used to spell out "foreign" words, so in addition to "konpyuta" (computer) and "amerika" (America, although this is a more recent name), you also have German words like "arubaito" (derived from the German word arbeit, to work) as well, as, for some reason, Chinese words like ramen (as opposed to the Chinese word, 拉面)

Chinese and Japanese approach foreign inventions and ideas in different ways. While Chinese attempted to accomodate for these words by modifying existing Chinese phrases (for instance, "diannao," or "electric brain," is the Chinese phrase for computer), Japanese uses katakana to represent that this idea is "foreign." Katakana itself was first developed in the Heian period (which came after the Nara period) and was originally used as abbreviations of various Chinese characters.

The American occupation changed a lot of things about Japan. For one thing, American culture and English became relatively popular as it was necessary for business. To give an example, the Japanese used to call America "Beikoku" (meaning rice country. Why). This was because the original way Japan dealt with foreign words was to adopt the Chinese character system of using various Chinese characters to approximate the sound of the word in whatever language. In this case, the "me" sound in America had the kanji for rice used.

However, after WW2, the education system changed how they adopted foreign words. Instead of basing them off Chinese characters, they would fully utilize katakana. This changed America into katakana directly. That is why when you go to Japan, the older people may refer to America as "beikoku" while the younger people refer to it as "Amerika."