Ok, this probably has been asked over and over but... why didn't Japanese Samurai use personal man-portable shields? In Europe, the Romans used their shields for all sorts of formations. Later in the Middle Ages, even chainmailed knights used shields. One big advantage of a shield is that it can block arrows while only adding a little bit of weight (since you only need to have the shield face the general direction of the arrows).
Japan was already covered but, as far as the Europeans are involved, this is not quite correct. While the knights initially (up to approximately XII century) were using the big shields, their sizes, with the improved limbs protection, had been steadily dwindling all the way down to a buckler (or no shield at all), which was absolutely useless against the arrows.
BTW, it does not look like the main purpose of the initial big shields of the knights was to protect against the arrows: while on a horseback, Knight simply could not held it in front of himself as a protection against crossbow bolts or over his head as a protection against the arrows shot at a high angle. The main purpose was to defend against opponent’s lance, hence the later Boucher style small shield with a special support for a lance. Not to mention that in a most of the continental Europe the archers and even crossbowmen were not numerous enough to became a critical factor in shaping knight’s equipment.
The big shields, pavisas, had been retained only by the special types of an infantry, mostly the shooting infantry like crossbow men and those with the early types of the firearms, as a protection against opponent’s fire: these troops were not expected to fight hand to hand combat. The infantry with the pole arms did not use them at all.
Ditto for the XV century depictions/reconstructions the French, Spain’s and German men-at-arms equipment and even the Mamelukes of the same period: no shields or very small ones. Not that a shield could be used simultaneously with a two-hand sword.
BTW, the archer-heavy warfare of the CA/Middle East used rather small shields that were useful in a hand to hand combat but hardly so as a protection against the arrows. If anything, the big shields (Scandinavian style) used in the Eastern Europe (specifically, Rus) during the early MA had been dwindling in size over the centuries of a constant contact with the neighboring nomadic archers and practically completely disappeared in the period following the Mongolian conquest. The 1st picture below - X -XI century, 2nd - XV - XVI:
(Hopefully, I did not exceed the pictures quota but it is better to see once than to read 10 times).