Jacob, Banner Lord of Wassenaer, Lord Obdam, Hensbroek, Spanbroek, Opmeer, Zuidwijk and Kernhem (1610, The Hague – 13 June 1665 off Lowestoft) was a Dutch Lieutenant-Admiral, and supreme commander of the confederate Dutch navy. The name Obdam was then also spelled as Opdam. British contemporaneous sources typically refer to him as Admiral Opdam or Lord Obdam because it was not until 1657 that he bought the Wassenaar Estate from relatives and thus acquired its title. Modern Dutch sources sometimes less correctly insert a second "van" between "Wassenaer" and "Obdam" or use the modern spelling "Wassenaar".

First Anglo-Dutch War
Jacob was born in 1610, the eldest son of Lieutenant-Admiral Jacob van Wassenaer Duivenvoorde and Anna van Randerode van der Aa. In 1631 he joined the army. On 28 April 1633 at Maarssen he married Agnes Renesse van der Aa. In 1643 he became drost (political governor) of Heusden, an important fortress town, and soon after military governor of its garrison.

As a member of the Hollandic nobility, Van Wassenaer was delegated to the States of Holland to represent their interests, as one of the ten members of the ridderschap (the "knighthood" Estate within the States). In 1650 when stadtholder William II of Orange died, he opposed installing the latter's infant son as nominal stadtholder. He bribed the nobility members of other States by promising them positions in the army. His opposition to the House of Orange was based on socio-economic and religious grounds: the stadtholders had their political base in the artisan class, which consisted mainly of puritan calvinists. Many members of the Van Wassenaer family were still catholic and feared religious oppression. When the First Anglo-Dutch War started in 1652 he, then a cavalry Colonel, was again delegated to the States-General. There he supported the faction of Johan de Witt who proposed to build a strong professional confederate fleet, at the expense of the army. Because his father had been an Admiral he was made "Delegate of the States to the National Fleet", thereby becoming responsible for all day-to-day dealings between the States-General and the navy, a position that carried much power.

Near the end of the war, in the Battle of Scheveningen, the supreme commander of the confederate Dutch fleet, Lieutenant-Admiral Maarten Tromp, was killed in action. His second in command had been Vice-Admiral Witte de With, both a courageous and competent sailor and a man seen as politically reliable as he wasn't a supporter of the Orangist faction. He would thus seem to have been the natural choice for a successor to Tromp. De With however also was a very quarrelous man who had made himself profoundly hated throughout the rank and file of the navy. His appointment might cause an immediate revolt. Third in command had been Vice-Admiral Johan Evertsen, again a brave and excellent sailor. Moreover he had much sympathy among the men. However Evertsen was commander of the Zealandic fleet. Hollandic captains would take offence to being subordinated to a man they had always seen as a rival. Worse, he had been a personal friend of the late stadtholder and was known to be an ardent supporter of the plan to make his infant son stadtholder. De Witt tried to find a more neutral candidate and offered command to Commodore Michiel de Ruyter. Much to De Witt's dismay De Ruyter declined. When even begging didn't help De Witt saw but a single solution to the deadlock: he ordered Van Wassenaer to take over command. The Colonel refused at first, protesting vehemently that he had no experience as a fleet commander or even as a captain. Political pressure became too great however and at last he consented.

New tactics
In 1654 the Dutch Navy had a new commander, Jacob van Wassenaer Obdam, Lieutenant-Admiral of Holland and West Frisia - and a complete amateur. This inexperienced man now had to solve the fundamental problem facing the Dutch fleet in that century: how to beat an enemy who was equipped with much more powerful ships. As the Dutch home waters were so shallow building very heavy ships was out of the question. De Witt had just convinced the States to spend four million guilders on a programme of sixty new warships but although these, carrying mostly about 44 cannon, were a lot heavier than the average Dutch ship of the last war, they were still little more than frigates by British standards. The typical solution when fighting Spanish galleons had always been the direct attack having the weather gauge, using superior manoeuvrability and numbers, or if that failed: employing fireships and boarding. Against the British however this was generally unsuccessful; they were at least as competent in these aggressive tactics and they had too many ships. Maarten Tromp then tried an informal line of battle, but this ploy came back with a vengeance. Robert Blake created a very formal version that worked even better for the British as they had very powerful ships and a more professional navy - the Dutch had to employ many armed merchants.

Studying Blake's Sailing and Fighting Instructions Van Wassenaer saw a new solution to the old problem. Now that a professional Dutch Navy was being created, this navy surely would soon be the equal in competence of the British one. That left only the inequality in firepower to be solved. He understood that this could be achieved by abandoning the traditional aggressive stance and embracing defence. Sailing in a battle line in a defensive leeward position, the wind, blowing from the side of the enemy, would give the guns of the Dutch ships a higher elevation and therefore a better range. That same wind would decrease the range of the enemy ships or even force them to close the gun ports of their lower gun deck - that carried the heaviest guns. So this became Van Wassenaer's favorite method: damage the enemy ships from a safe distance and then disengage. Whether the enemy was destroyed or your own fleet damaged too was immaterial. With their superior shipbuilding capacity the Dutch could always make quicker repairs. Simply keeping the enemy fleet inoperational would suffice. Dutch trade wouldn't be disturbed and while a few battles might well deplete the enemy's treasury, the Republic would always have plenty of reserves. In Van Wassenaer's opinion naval warfare was a gigantic battle of attrition that the Dutch were guaranteed to win.

Northern Wars
In 1655 Charles X of Sweden started a series of aggressive campaigns (part of the Northern Wars) intending to make Sweden the dominant power in the Baltic. The Dutch saw this as threat to their vital interests. Although they are today better known for their exploitation of the East Indies, in fact their Baltic trade was more profitable in absolute terms. Also the Republic was critically dependent on Scandinavian wood to build ships and Polish grain to feed its large urban population.

When Charles conquered Poland, the Dutch supported the subsequent rebellion and sent Van Wassenaer with a fleet to relieve Danzig in 1656. In 1657 Van Wassenaer blockaded Lisbon and captured fifteen ships of a Portuguese sugar fleet, but in 1658 had to return to the Baltic as the situation there had grown even more critical. After the failure of his Polish campaign Charles had turned his attention on Denmark and had invaded Jutland from Germany. He then made peace with Frederick III of Denmark but treacherously broke it a few weeks later in an attempt to take Copenhagen by assault. This failed and he laid siege to the Danish capital, the last part of his kingdom still under Frederick's control.

After much deliberation the States-General decided to send the entire active Dutch fleet and a mercenary army to reliev