OK, that's a bit of an exaggeration. Japan, under its 1947 constitution has always had multiple political parties. But one of them has been very dominant.
A significant leader of early postwar Japanese politics was Shigeru Yoshida. He was prime minister from 1948 to 1954, and many credit him with putting the country on its path to recovery. His Liberal Party, lacking a majority in the House of Representatives, lost a confidence vote in 1954, and Ichiro Hatoyama of the Democratic Party succeeded Yoshida. But that kind of rivalry and confrontation soon came to an end.
The Liberals and the Democrats shared a center-right orientation, and were opposed by the Socialists. The two right-wing parties merged in 1955, to present a united front against the Socialist opposition. After having thus staked out a large swath of the political spectrum, the Liberal Democratic Party (LDP) constituted the governing party of Japan, continuously until 1993.
The Japanese electorate's opinion of the LDP blew hot and cold over the years, but they were never sufficiently disillusioned to reach far enough left to grant power to the Socialists, who formed the main opposition to the LDP for many years.
What political competition there was in Japan in those years, consisted of rivalry between factions of the LDP. Each of those factions has been centered around a particular political boss. Apparently, they have always reflected the competing ambitions of their leaders, rather than ideological differences.
In some ways, those factions have functioned like separate parties, and the LDP can be seen as more of a coalition government than a party, per se. However, voters did not generally have a clear choice between factions, in the manner that, for example, an American voter has in choosing between a Republican and a Democrat for Congress. LDP politicians' resultant lack of accountability may bear some responsibility for various corruption scandals over the years.
LDP power was also based in part on a phenomenon similar to one that prevailed in American politics until the 1960s: overrepresentation of rural areas. Some U.S. states failed to redraw district lines for the U.S. House and state legislatures for several decades. Therefore, urban and suburban areas, whose population had grown in the meantime, ended up underrepresented in those legislative bodies. The U.S. Supreme Court put at end to that, with a series of decisions beginning in the mid-1960s.
In Japan, that pattern persisted beyond the '60s and, to a limited extent, continues today. By tailoring its policies to the interests of the rural population, the LDP was able to further cement its hold on power.
Late in the 20th century, some factions of the LDP broke totally away, and started rival parties. The LDP weathered that development reasonably well for several years.
But there was a general election in 1993 in which no party won a majority of seats in the House of Representatives. For a brief period, there was a coalition government that excluded the LDP. However, due to a shifting of party coalitions, the LDP was back in power as the leader of a different coalition government the following year. And the LDP again won a majority of seats in the 1996 general election.
Then, in 1998, a development occurred that could, later this month, result in the LDP's fall from power, maybe permanently. It was in that year that the Democratic Party of Japan was founded. More about that organization, and the setting for the general election to be held on August 30, in future posts.