Some would describe taxation as a form of theft, and conscription as a form of slavery — in fact some would prefer to describe taxation as slavery too, or at least as forced labour.
It appears Nagel himself disagreed with this position. Nevertheless, the idea that taxation involves the involuntary removal of an individual’s property, and hence is comparable to theft, is a valid one.
Where tax paid amounts to half of a person’s income, that person effectively has to work, involuntarily, as much for the government as for himself.
* 'Ruthlessness in public life', reprinted in Thomas Nagel, Mortal Questions, Cambridge University Press, 1979.