Is it wrong to withhold taxes to protest expenses on things like military weapons?
In this question, we are considering the moral obligation of the individual to participate in a system of distributive justice within our society, as outlined in civil tax law. To deal immediately with your example of the state spending money on military weapons, it is generally agreed that it has an obligation to protect its citizens. This is a grave duty of those who hold lawful authority. They have the right to use arms to repel aggressors in a responsible manner.
In his letter to the Romans (13:1-2), Paul exhorts us to "pay....taxes to whom taxes are due". The Catechism of the Catholic Church (no. 2240) states that "submission to authority and co-responsibility for the common good make it morally obligatory to pay taxes". It is commonly agreed by moral theologians that we are obliged in conscience to pay taxes. We are morally obliged to contribute to the common good of society. We have a duty to contribute to the advancement of our society and to support our leaders in working for the good of society in a spirit of truth, justice, solidarity and freedom. The fourth commandment requires each of us to respect and obey those who exercise legitimate authority in our country.
Interestingly, in our historical tradition, many have held that tax laws oblige us in commutative justice, based on a presumed contract between the individual and those who govern our society. In this sense, taxes are paid for the services rendered by the authorities. Then, with the development of hierarchical Catholic social teaching by Pope Leo XIII in the nineteenth century and the simultaneous importance given to Thomistic thought, most Catholic commentators grounded the obligation to pay just taxes in legal or social justice, which governs the individual's relationship to the society in which he or she lives.