Matsya Purana 240.5

240.5

aśuddhapārṣṇinṛpatirna tu yātrāṃ prayojayet.
pārṣṇigrāhādhikaṃ sainyaṃ mūle nikṣipya ca vrajet .. 240.4 ..

Matsya said:—A king should march when he finds his enemy pressed by some valiant king or his tributary chiefs. He should first see that he has abundance of strength and fighting men and materials and then he should leave a powerful guard to protect his stronghold before starting to face his enemy, without which he should never set out. To protect his own country, the base of all operations, there should be many more fighting persons and generals than the other kings and chiefs engaged in conquering his enemy. He should never start when his tributary chiefs and the neighbouring kings are not subject to him. It becomes very hot during the month of Chaitra so Chaitra should be chosen for an expedition as far as possible, or the month of Agrahayana when rains cease might be chosen otherwise he should take every advantage of his enemies' adverse circumstances.—2-6.