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The phrase to throw away the scabbard, and its variants, mean: to abandon all thought of making peace.

This phrase occurs, for example, in the following from The sword is drawn, by Conrad Black, published in the National Post (Toronto, Ontario, Canada) of Saturday 31^st August 2019 [page A15, column 5]—the British politician Boris Johnson (born 1964), then Prime Minister of the United Kingdom and Leader of the Conservative Party, was making preparations for a no-deal Brexit:

Johnson has thrown away the scabbard, and Brussels and, more relevantly, Paris and Berlin can be in no doubt that the chicken game is over: they can make serious concessions to the U.K. or suffer the defection of Europe’s second economy.

These are, in chronological order, the earliest occurrences of the phrase to throw away the scabbard and variants that I have found:
—Note: This phrase originally occurred in the proverb: who draws his sword against his prince, must throw away the scabbard and variants:

1-: From Eight Military Aphorismes, demonstrating the Uselesness, Unprofitableness, Hurtfulness and Prodigall Expensiveness of all standing English Forts and Garrisons, to the People of England [&c.] (London: Printed for the Author, 1658), by the English author William Prynne (1600-1669) [page 31]:

Machiavills Atheisticall infernal Paradox (applauded and oft justified in Print by his new Disciple Politicus) He that hath once drawn his sword against his lawfull Soveraign, must throw away the scabbard, and never sheath it more; no, not after he hath cut off his Soveraigns Head therewith.

2-: From Lexicon Tetraglotton, an English-French-Italian-Spanish Dictionary (London: Printed by J. G. for Samuel Thomson, 1660), by the Welsh author James Howell (c. 1594-c. 1666) [English Proverbs, page 17, column 1]:

VVho draweth his sword against his Prince, must throw away the scabbard.

3-: From The History of Independency. The Fourth and last Part. Continued from the Death of his late Maiesty, King Charls the First of happy Memory, till the deaths of the chief of that Juncto (London: Printed for H. Brome and H. Marsh, 1660), by “T. M. Esquire, a Lover of his King and Country” [page 54]:

Tis the custom of rebells to go on from bad to worse, and when they have once drawn the sword against their Prince, to throw away the scabbard, and never entertain a thought of return or repentance.

4-: From Pindarique Odes, Written in Imitation of the Stile & Manner of the Odes of Pindar (London: Printed for Henry Herringman, 1677), by the English poet Abraham Cowley (1618-1667) [page 51]:

Note 2. As not intending to put it up again, or to be ever reconciled; in which sence [sic] it was said, as I take it, to the great Duke of Guise, that he who draws his Sword against his Prince, should fling away the Scabbard.

—Note 2 refers to the following passage from The 34. Chapter of the Prophet Isaiah [page 48]:

I see the Sword of God brandisht above;
And from it streams a dismal ray;
I see the Scabbard cast away.

5-: From The History of the Civil Wars of France ([London]: Printed by T. N. for Henry Herringman, 1678), a translation of Historia delle Guerre Civili di Francia, by the Italian historian Arrigo Caterino Davila (1576-1631) [Book 9, page 344]—context: “The Duke of Guise seeing the City in his power, and the King as it were a prisoner, ceaseth to prosecute the forceing of the Louvre, and appeaseth the people”:

Many thought, and particularly Alessandro Farnese Duke of Parma, (a Prince of incomparable valour and deep understanding) said, that the Duke of Guise had attempted too much, and done too little, not remembring [sic] the Proverb, that Whosoever draws his Sword against his Prince, ought presently to throw away the Scabbard: for so bold an enterprise should either not have been undertaken, or being begun, should have been executed, whatsoever had come on it.

6-: From A Sermon preached in the Church of Putney in the County of Surrey, upon the 24^th of April, 1681. His Majesty’s Declaration being read that day (London: Printed for R. Horne, 1681), by “Edward Sclater, M. A., Minister there” [page 28 (erroneously numbered 18)]:

The great Master of crude mischiefs, whose design it always was to animate a People against God and his Anointed, has in all Ages taught his Disciples this Maxim of Hell’s Polity, that he who once draws his Sword against his Prince, must throw away the Scabbard, and never so much as think of sheathing it again.

7-: From The History of Popish-Sham-Plots from the Reign of Queen Elizabeth to this Present Time. Particularly of the Present Popish Plot. Being an Account of the several Methods the Papists have used to stifle it [&c.] (London: Printed for Richard Janeway, 1682) [chapter 2, page 12]—the following is about the execution of Charles I (1600-1649), King of England, Scotland and Ireland (1625-1649):

In the House of Commons […] there were hardly so many Non-conformist Ministers, Presbyterians, Independents and Anabaptists altogether, as there are Counties in England. And they were Church of England Men that first drew the Sword, (though indeed some of the Sectaries afterwards threw away the Scabbard.)

8-: From A Sermon preached before the Artillery Company of London at St. Mary Le Bow, April 20. 2682 (London: Printed for John Baker, 1682), by “Thomas Sprat D.D. One of His Majesties Chaplains in Ordinary” [page 28]:

This cheerful posture, and prepared readiness of your Swords to be drawn, will be an abundant Safety to our King, and Country without once drawing them. Such will be the Innocence, as well as Justice of your Arms, such the desperate condition of your Adversaries; that whenever they draw their Swords against their Prince, they must throw away the Scabbard: whilest yours, by God’s Providence, being manag’d with an ordinary watchfulness, and sobriety, will be sufficient to defend him, without ever unsheathing them.

9-: From The Trimmer, His Friendly Debate with the Observator concerning Uniformity; Rendition of Charters, the House of Commons, not a House of Courtiers [&c.] (London: Printed for R. Hall, 1683) [chapter 6, page 14]:

He that uses a Sword in a Court of Justice, must throw away the Scabbard.