From Sega Retro
Thunder Force is Tecnosoft's flagship series, started in 1984 with the Sharp X1 release of Thunder Force (which was eventually ported to other Japanese home computers of the time), and followed by the X68000 Thunder Force II. II would be ported to the Sega Mega Drive, and its success guaranteed future games appearing on the Mega Drive, turning it into the flagship shoot-'em-up series on the Mega Drive too. These later three games were eventually bundled into "Gold Packs" for the Sega Saturn before seeing its own new game. Thunder Force itself has never been remade after the initial ports, let alone appeared on a Sega system.
Each game has the Galaxy Federation either fighting the ORN Empire or one of the alliances formed by its survivors after being annihilated at the end of III. I and II used an overhead view where you needed to navigate a field to destroy targets; II also had standard side-scrolling levels which would become the standard with III. The ship in each game has four freely selectable speeds and a number of upgradable weapons on top of the already upgradable two standard ones (shoot two shots forward or one shot forward and one shot back); intelligently switching between all of these is the key to victory.
After Tecnosoft was bought by Twenty-one, material for a planned Thunder Force VI was released to the public and a group of enthusiasts set out to make a doujin sequel of their own. "Broken Thunder" was a notoriously buggy failure, eventually being pulled from store shelves. Tecnosoft would then go back to Sega to make a real Thunder Force VI, released in 2008, though this would be developed entirely from scratch by a team at Sega with little to no involvement from Tecnosoft.