Frederik Johan van Baer, Lord of Slangenburg (27 July 1645 – 15 December 1713) was a Dutch States Army officer.

Slangenburg didn't join William III during his invasion in England, but served with the Allied army in the Spanish Netherlands.

When it became clear that William III would not give him his way in this, he threatened to resign and furiously announced that both Emperor Leopold and the Elector of Bavaria were vying for his favour.

Slangenburg eventually effectively resigned from the States Army in 1690, but two years later, at William III's intercession, he was retroactively appointed lieutenant general of infantry.

There, he successfully averted the encirclement and destruction of a Dutch division, who Obdam had manoeuvred into a dangerous position, despite warnings from his Slangenburg and Tilly.

During the battle, Obdam was forced to flee and hastily retreated to Breda with a small contingent of soldiers, after which the command fell on Slangenburg.

Outnumbered by at least two to one the fierce fighting claimed the lives of over seven hundred Dutch soldiers (as well as a considerable number of French and Spanish troops), ultimately resulting in an inconclusive outcome.

Dubbed the 'hero of Ekeren,' Van Baer managed to send several captured standards, banners, and kettle drums to The Hague, where they were displayed as symbols of triumph in the great hall of the Binnenhof.

During the allocation of commands for subsequent operations in Flanders in 1703 and the recommendations for promotions following the campaign's conclusion, he felt overlooked in favor of officers he deemed less qualified.

While his arguments were occasionally valid and his insights often accurate, his strong desire for recognition, lingering resentment, inflexibility, and the fervor with which he expressed his opinions increasingly irritated, especially the English ally.

[2] On April 11, 1704, Van Baer was promoted to the rank of General of Infantry and appointed as the commander of the Allied army on the Moselle.

He wrote: ... if the valiant Republic, to whom Protestant civilization owes an inestimable debt, was to be deprived of its fruition in modern times, condemned for ever to be a minor Power while rivals grew so great, this was the fatal scene.

Here by the cross-roads of bodeful Waterloo, as earlier upon the heaths of Peer, the destinies of Holland turned; and upon that milestone there may well be inscribed the not otherwise noticeable name of Slangenberg.

According to Knoop, despite his purported intentions Marlborough failed to undertake the requisite measures to prepare his army for combat, writing: Little chance of winning; great chance of suffering severe losses in defeat; - that double consideration should have deterred from engaging in battle, and we are fully convinced that Marlborough did not desire the battle.