He's referring to George III's Speech from the Throne (the traditional regal address at the State Opening of Parliament) on 27 October 1775, which followed and expanded upon the earlier Proclamation of Rebellion. The speech was essentially a declaration of war; George declared that he would use military force to suppress the rebellion and restore British control of the Thirteen Colonies:

The rebellious war now levied is become more general, and is manifestly carried on for the purpose of establishing an independent empire. I need not dwell upon the fatal effects of the success of such a plan. The object is too important, the spirit of the British nation too high, the resources with which God hath blessed her too numerous, to give up so many colonies which she has planted with great industry, nursed with great tenderness, encouraged with many commercial advantages, and protected and defended at much expence of blood and treasure.

It is now become the part of wisdom, and (in its effects) of clemency, to put a speedy end to these disorders by the most decisive exertions. For this purpose, I have increased my naval establishment, and greatly augmented my land forces; but in such a manner as may be the least burthensome to my kingdoms.