Self defense of Monks and Nuns who are still weak in The Dhamma falls under the rulers, all rulers are required to have warriors and an army:
A ruler is required to provide protection to both people and animals in the kingdom:
AN, Book of 3:15
“In this case, the world ruler, the just and righteous king, relying on the law of righteousness (Dhamma), honouring it, regarding it highly and respecting it, with the law of righteousness as his standard, banner and sovereign, provides lawful protection, shelter and safety for his own dependants. He provides lawful protection, shelter, and safety for the warrior-nobles attending on him; for his army, for the brahmins and householders, for the inhabitants of town and countryside, for ascetics and brahmins, for the beasts and birds.
“A world ruler, a just and righteous king, who thus provides lawful protection, shelter, and safety for all, is the one who rules by righteousness only. And that rule cannot be overthrown by any hostile creature in human form.
Also, no army deserters for monk hood, they must serve and protect first:
From The Mahavagga of The Vinaya:
http://www.sacred-texts.com/bud/sbe13/sbe1312...
4. Then the Magadha king Seniya Bimbisâra went to the place where the Blessed One was; having approached him and having respectfully saluted the Blessed One, he sat down near him. Sitting near him the Magadha king Seniya Bimbisâra said to the Blessed One: 'Lord, there are unbelieving kings who are disinclined (to the faith); these might harass the Bhikkhus even on trifling occasions. Pray, Lord, let their reverences not confer the pabbaggâ ordination on persons in royal service.'
Then the Blessed One taught (&c., see chap. 39. 7, p. 196 down to:), thus addressed the Bhikkhus: 'Let no one, O Bhikkhus, who is in the royal service, receive the pabbaggâ ordination. He who confers the pabbaggâ ordination (on such a person), is guilty of a dukkata offence:
The Kosiya Jataka advises King Pasenadi on timeliness of war:
Here Lord Buddha said, there is a time for everything including war, everyone should abide in practicing Dhamma and strike when the time is right:
No. 226.
KOSIYA-JatAKA.
[208] "There is a time," etc.--A story told by the Master at Jetavana, about the king of Kosala. This king started to quell a border rising at a bad season of the year. The circumstances have been described already 1. The Master as before told the king a story.
_____________________________
Once on a time, the king of Benares having started for the field of war at an unseasonable time, set up a camp in his park. At that time an Owl entered a thicket of bamboos, and hid in it. There came a flock of Crows: "We will catch him," said they, "so soon as he shall come out." And they compassed it around. Out he came before his time, nor did he wait until the sun should set; and tried to make his escape. The crows surrounded him, and pecked him with their beaks till he fell to the ground. The king asked the Bodhisatta: "Tell me, wise sir, why are the crows attacking this owl?" And the Bodhisatta made answer, "They that leave their dwelling before the right time, great king, fall into just such misery as this. Therefore before the time one should not leave one's
p. 147
dwelling place." And to make the matter clear, he uttered this pair of verses:
"There is a time for every thing: who forth from home will go
One man or many, out of time, will surely meet some woe;
As did the Owl, unlucky fowl! pecked dead by many a crow.
"Who masters quite each rule and rite; who others' weakness knows;
Like wise owls, he will happy be, and conquer all his foes."
[209] When the king heard this, he turned back home again.