If your definition of "theft" is "taking something by (threat of) force", then (1) private property at least of land clearly is theft, (2) one could even go further and argue that some forms of taxation
aren't theft, and (3) police are regularly involved in theft even while doing the right thing.
The key is in "taking something": there is a baseline state of the world against we're comparing the current status quo.
For point (1), there is clearly a historical baseline in which no land was owned by anybody, and all land was equally accessible to all. Then people started to make claims that some part of land was theirs, and that they had the right to exclude others from it - if necessary by force. They took something from others by (threat of) force - hence committing theft according to your definition, and this theft is enshrined by modern property law.
For point (2), consider the way income taxes work in many (most?) countries. Instead of money being paid by / taken from the employee, the money is instead withheld. The money that is "paid as income tax" is in reality never the employee's to begin with, so it is not taken away from the employee - hence no force is involved.
You might argue that the money is taken from the employer, but nobody forced the employer to employ someone.
For point (3), consider what happens when person A owns some widget, but person B enters their house at night and takes it from them - without force, because person A was asleep. Note first that, that is not theft according to your definition! Later on, person B is tracked down by police, who arrest him or her by force to return the widget to person A. The police have just engaged in theft - which is totally absurd use of language, but a necessary consequence of how you want to define theft.
Points (2) and (3) highlight something important: what belongs to whom is always a social construction. Social convention (encoded into law) says that land ownership is possible, and therefore land ownership is not actually theft. Social convention says that police are allowed to take stolen goods back to their original owner, and therefore those actions aren't theft. And whether you like it or not, social convention also includes taxes, and that is why taxes aren't theft. Because at the end of the day, the useful definition of theft has nothing to do with force - the useful definition of theft is "taking something against the established rules of society".