At the end of the Cold War Japan had a small but well equipped and trained air force. But since 1991 there was not a lot of enthusiasm in parliament or among voters to spend what was needed to keep the air force up-to-date. Since 2000 it became more obvious that China was building a modern, and threatening, air force. Russia was becoming more hostile as well and North Korea remained a problem. Gradually Japan accepted the fact, and cost, of upgrading its air force.
The most recent example of this is Japan finally acting on long-delayed plans to upgrade 98 of its heavily used F-15J interceptors. The parliament was reluctant to spend the billions of dollars this would cost. For some legislatures it was an economic decision, for others it was feared upgrading would anger China and Russia. But as incursions by Chinese and Russia military aircraft increased fivefold over the last decade, to over a thousand a year, parliament changed its mind. The upgrades will cost $4.5 billion and be carried out in Japan using American made electronics. This includes an AESA radar, new computers and EW (Electronic Warfare) equipment. This includes a jam-resistant GPS device. There will also be new communications including jam-resistant digital data links.
The growing presence of Russian and (mostly) Chinese military aircraft has also persuaded legislators to spend a lot of money replacing older aircraft. The Japanese Air Force has about 50,000 personnel and 775 aircraft, 39 percent of them fighters. The air force is usually the first responder when any potentially hostile ships or aircraft come near any of the islands. Most of the 302 fighters are in need of upgrades or replacement. The 73 F-4s were built in Japan during the 1970s and are very much in need of replacement despite light use, some upgrades and careful maintenance. The new F-35s are initially being used to replace retiring F-4s. The 155 F-15Js were also built in Japan but during the 1980s and 90s. These are the most heavily used warplanes as most serve as interceptors. There have already been some upgrades but the newly ordered upgrades are the most extensive in a while. The 62 F-2s are a Japanese version of the F-16, developed and built in Japan between 1995 and 2011. There are currently 12 F-35s in service out of 147 on order. Some (38 F-25As) are being assembled in Japan and additional orders for F-35As will probably also be built in Japan.
In late 2018 Japan decided to order another 99 F-35 fighters. This will cost about $15 billion, spread over nearly a decade of annual defense budgets. Most will be the land-based F-35A model but as many as 40 will be F-35Bs, the version that can operate from carriers. Japan already had 42 F-35As on order to replace 73 F-4 interceptors. The new F-35As on order are to replace a hundred older F-15J fighter-bombers.
There may be more Japanese F-35 orders because its locally designed F-2 maritime attack aircraft has been canceled on account of high cost and uncertain performance. Japan also sought to design and build a stealthy replacement for the 1990s era F-2 fighters but concluded it would be too expensive for just a hundred aircraft and such an “X-2” aircraft could not compete against the American F-35 in export markets, assuming Japan changed its constitution to allow weapons exports. So now Japan is planning to eventually replace the F-2 with F-35s. Japan imports a lot of foreign warplanes but usually assembles them locally under license, which it will do with the F-35s.