In 1629 George raised a troop of horse for Swedish service in Germany during the Thirty Years' War and became a major in Colonel James Spens' regiment.

After promotion to lieutenant-colonel he returned to England, by March, and having collected a regiment of foot, he obtained Chancellor Axel Oxenstierna's support to return to Sweden to meet King Gustav II Adolf personally regarding military matters.

It is said that after the fall of Gustav II Adolf he held back retreating Swedish troops together with the King's court and army-chaplain, superintendent Dr. Jacob Fabritius.

[3] His account of King Gustav II Adolf's death: The King at the first charging to the graft, was shott through the arme and his horses neck; upon which they would fayne have perswaded him to have retyred, which he refused, rideing to the heade of the right winge of horse, incourageing them, saying nothing ailed him, and with that, comandeing them all to follow him, he leaped over the graft and charged the enimie; but there followed him but fower regiments, which weare encountred with soe many of the enimie that they wre forced to retyre.

There fell iust by him one Truckes his chamber younker, whoe was brought off alyve, but since deade: he reported that when our horse retyred there came an officer of the enimies, whoe asked him whoe the Kinge was, which he refused to tell; he shott him through the bodye, and with that went to the Kinge demanding of himself what he was, to which he replyed he was the Kinge of Swethen; upon which he thought to have carryed off the bodye, but seeing our horse comeing againe to charge, he stroak the Kinge through the body with a broade sworde twyce, and then runn away, for then Herticke Bernard himself charged with the horse, beat the enimie back and brought the corps off, which had receaved in all tenn woundes.

The Swedish Chancellor Axel Oxenstierna, seeking a closer alliance and further recruits, sent Fleetwood to the Stuart court in 1636 and 1637.

In 1639 he sailed with Covenanters through the Danish Sound to take up service first as a commandant at Greifswald and then at Kolberg the following year.

In 1655 he was sent by Charles X as envoy extraordinary to Oliver Cromwell, in response to Bulstrode Whitelocke's embassy.

He was accompanied by his eldest son, Gustavus Miles Fleetwood(1642-1705), who was enrolled among the life-guard of Charles II, and pursued his education in England.

This was at the same time as Alexander Leslie[5] and William Cranston[6] were organizing levies of Royalists for Swedish service.

[7] SWEDEN, JAMES SPENS (STRALSUND Mecklenburg-Vorpommern) Arrived 1629-07-01, as MAJOR Departed 1632-12-31, as LIEUTENANT- COLONEL Capacity OFFICER, purpose MILITARY