Although full of actors with a lot of experience in westerns--Jim Davis, Harry Lauter, Lee Van Cleef, Douglas Fowley--this cheap and irritating film from Republic comes across like it was made over a weekend by a couple of guys who got some money (very little of it, from the looks of things) together and said, "Hey, let's make a western!". This was released by Republic near the end of its existence--the studio went out of business two years later--although it wasn't actually made by them but was an independent production they picked up for distribution. That at least salvages their reputation somewhat, since Republic specialized in making westerns and knew how to make efficient, action-packed horse operas; I can't think of any of its own product that is as tenth-rate as this mess is.
There are so many things wrong with this film that it's difficult to know where to begin. For starters, although it's called "Raiders of Old California", it's set along the Texas/Mexican border, nowhere near California. As pointed out by other reviewers, the US troops wear uniforms and equipment that weren't issued until 15 years or so after the period the film was set in (the late 1840s). The film opens during the Mexican-American War of 1848 with a US Army attack on a Mexican fort, and it's an indication of what's to come--the "action" is dull, slow, poorly staged and full of stupid mistakes (while attacking the fort, the US soldiers don't bother running but stand out in the open, where they are promptly shot; soldiers fall off their horses although no shots are heard being fired; after the battle is over and the Americans have taken the fort, a Mexican soldier rides through the front gate and starts speaking to his commanding officer--in English--apparently not noticing that the fort has been taken over by American soldiers), and the "fort" itself is a painfully obvious, shoddily made set that looks like it was slapped together with wrapping paper and plywood.
The story of greedy villains trying to take land away from poor defenseless peasants and farmers has been done a thousand times before (and a thousand times better) and despite the cast of western veterans, no one acquits themselves particularly well. I hope their checks didn't bounce so at least they got something out of it, because this flabby, badly written, sloppily made hackjob isn't anything any of them should be proud of.