[Alt History] A cavalry man, standing by.

The Cavalry corps were reconstituted after Puerto Rico became a republic, originally the corps were mostly armed with swords and lances but as time went on the military started to shift towards rifles and pistols while leaving swords and lances relegated to ceremonial duties and parades. By the early 1900's the cavalry only issued sabers to officers, a lot of the sabers would end up being transferred to the navy who would put them into the "Sable Modelo 1900", also known simply as M19 (They were shortened) while those that remained in cavalry service were used by the Presidential Guard and by members of the cavalry who were out on parade. As for the lance it remained in service until 1942 and was used in the early days of the conflict by the cavalry who reported great success.
Uniforms wise the uniform was the same as the one issued to the infantry with the exception of footwear and headgear, Puerto Rican cavalry made use of a helmet that before the war was known for it's shine and plume but by the end of 1916 the plume had been removed and it's shine replaced with a matte black or matte brown paint, some members of the cavalry continued to put a feather on their helmets while others did not, the cavalry's helmet also showed the military leadership the need for helmets for all soldiers since cavalry reported less deaths from head wounds.

During the early days of the war the Cavalry made great use of its lances, shock troops essentially as they charged infantry from the side, lances at the ready to pierce their enemies, this however would not last and the introduction of the tank into the battlefield would put an end to such engagements, after all horses were needed to move supplies from the train stations to the front and the field of battle had changed from open engagements to trenches and mud.  By the middle of 1917 most of the cavalry was carrying out front line duties in the trenches while a small group continued to carry out hit and run attacks against supply lines and advancing troop movements. The last actual cavalry charge would happen on the 27th of March 1918, the cavalry would charge downhill against advancing US infantry, Captain Justino Vargas who was short on ammunition ordered the men under his command to ready bayonets and charge.