The Battle of San Juan Hill is best known for the charge of the 1st U.S. Volunteer Cavalry, famously called "the Rough Riders," led by Col. (and future President) Theodore Roosevelt. However, there was much more to that battle than the single, iconic charge. In fact, by some accounts, the attack was what they call a Charlie-Fox. But if it happened today, would it be the same, nail-bitingly close battle?
Historically, the Battle of San Juan Hill pitted 800 Spanish troops on strategically important heights outside Santiago, Cuba, against 8,000 American troops and 3,000 Cuban insurgents. Back then, the Spanish had the advantage of more modern rifles, machine guns, and artillery. So, for the sake of argument, we'll call it roughly two light infantry battalions against two infantry brigade combat teams. As we talk about a hypothetical, modern Battle of San Juan Hill, we'll also leave out drones and air support – just to try and keep this comparison "apples to apples."
Today, in terms of small arms, the United States has the advantage. Spain uses the Heckler and Koch G36, a rifle that the Germans designed but are now dropping due to its myriad problems.
German troops with G36 rifles carry out a demonstration during BALTOPS 2004. Spain also uses that piece-of-crap rifle.
(US Navy photo by Photographer's Mate 2nd Class George Sisting)
The United States, on the other hand, uses the M4 carbine and M16 rifle, which are much more reliable and accurate. Most of the other weapons in service are roughly equal, with the exception of Spain's M109A5 self-propelled howitzers, which are less modern than American M109A7 Paladins. In terms of munitions, United States has the advantage of the Excalibur GPS-guided shell.
Today, the "Rough Riders" under Teddy Roosevelt's command would enjoy the edge in small arms and artillery that the Spanish had in 1898.
Today, Spain no longer has the technological advantages they once enjoyed. The U.S. simply has better rifles and artillery at their disposal, which would change the entire dynamic of the battle. First off, American artillery would be able to deliver much more suppressive fire in the 2018 Battle of San Juan Hill.
The United States Army's M109A7 Paladin howitzers would bring a decisive edge against Spanish artillery today.
But the real difference lies in American rifles. In the historical battle, the Spanish held out against overwhelming numbers, inflicting about 1,300 casualties on the Americans, due to a combination of defensive positioning and more modern weaponry. This time around, the Americans would make the charge with top-of-the-line weapons while artillery keeps the Spanish holed up.
In short, it'd be a rout. What was once a daring, uphill charge would feel more like a casual stroll.