In the event of or imminent danger of war, insurrection, rebellion, invasion, tumult, riot, resistance to law or process or breach of the peace, if the Governor shall have ordered into active service all of the available forces of the National Guard of South Carolina and shall consider them insufficient in numbers to properly accomplish the purpose, he may then in addition order out the unorganized militia or such portion thereof as he may deem necessary and cause them to perform such military duty as the circumstances may require.