What are the powers of the king?

Question

In Judaism was the King an absolute monarch or did he have limits on his authority.

I have heard that Solomon had Joab executed even after the Sanhedrin acquitted him.

Answer 1

Maimonides codifies the rules relating to the king in detail in Hilchot Melachim. There were certainly some limitations to his authority, some examples of which follow. (All quotes are from the Touger translation, all emphases are mine.)

Hilchot Melachim 3:8

The king may only execute people by decapitation. He may also imprison offenders and have them beaten with rods to protect his honor. However, he may not confiscate property. If he does, it is considered theft.

Hilchot Melachim 3:9

Needless to say, if a king decrees that a mitzvah should be negated, his words should not be heeded.

Hilchot Melachim 4:3

Similarly, he may take all those that are necessary for him from the nation's craftsmen and employ them to do his work. He must pay their wages. He may also take all the beasts, servants, and maids that are necessary for his tasks. He must pay their hire or their value as ibid.:12-16 states: 'He will set them to plough his ground and to reap his harvest, to make instruments of war, and gear for his chariots.... He will take your servants, your maids, your finest young men, and your donkeys to do his work.'

Hilchot Melachim 4:6

He may take fields, olive groves, and vineyards for his servants when they go to war and allow them to commandeer these places if they have no source of nurture other than them. He must pay for what is taken. This is stated in ibid.:14: 'He shall take your good fields, vineyards, and olive groves and give them to his servants.

Hilchot Melachim 5:2

There is no need to seek the permission of the court to wage a milchemet mitzvah. Rather, he may go out on his own volition and force the nation to go out with him. In contrast, he may not lead the nation out to wage a milchemat hareshut unless the court of seventy one judges approves.

Answer 2

The Talmud says:

The king or the High Priest may not be members of the [Sanhedrin's] board for [deciding when to add a month to the year]: The king on account of the upkeep of the army, and the High Priest because of the cold. [Sanhedrin 18b]

Here is what it means. Before the permanent calendar, the Sanhedrin had to decide which year would be leap year (13 months) and which regular (12 months). The king was presumed biased in favor of adding a month, because he paid his soldiers by the year, so adding a month makes the year longer! The High Priest was presumed biased against adding a month, because he has to immerse five times in the mikvah on Yom Kippur, and if Yom Kippur falls a month later the water is colder! This is why they were both excluded from participation. [Yoma 31b]

These are very human considerations!