what he knew before anyone else

Due to his investment in the outcome of the war and the creation of the network for war communications, Nathan Mayer Rothschild received news of Britain’s victory at the Battle of Waterloo far before any other groups, even Westminster itself.

Even though Nathan's communication system was utilized as the primary infrastructure for the British government, he was still a middle man, receiving the news first, then having it relayed to Westiminster. 

In 1815, the combined forces of Britain and Prussia defeated Napoleon at the Battle of Waterloo, ending the Napoleonic Wars. In light of this immensely important news, “on the night of 19 June 1815...a courier arrived at New Court bearing the news of Wellington’s victory at Waterloo [to Nathan], a full 48 hours before the government’s own riders brought the news to Downing Street.”

This process can be summarized as "a Rothschild agent, at the battle, jumped into a boat at Ostend. [After arriving by horseback], Nathan Rothschild let his eye fly over the lead paragraphs." 

At this time, Nathan is the only individual in London with the knowledge of the battles outcome. That is powerful knowledge, however, originally, Nathan didn't bother to keep this information a secret from the British government. Just a moment later he was "on his way to to tell the government that Napoleon had been crushed. But, [incredibly], his news was not believed, because the government had just heard of the English defeat at Quatre Bras."

While Europe awaited the official results of the battle of Waterloo in 1815, Nathan Rothschild already had the news that the English military forces had won. 

He used this information to his own advantage