In the summer of 1944, the big push in the Pacific was the invasion of the Philippines. Securing Palau and the Marianas was deemed prudent, to prevent Japanese planes or ships from striking the rear of the U.S. invasion fleet or striking at post-invasion shipping. The invasion plan actually called for bypassing the largest garrisons in these areas. Peleliu and Anguar were smaller islands southwest of Palau, which the Japanese had controlled for 30 years.

Reconnaisance began in earnest in June 1944. U.S. Navy frogmen quietly examined beach defenses on Peleliu, the island was photographed from the air, and captured documents from Saipan were digested. The U.S. military learned the two islands were more heavily defended than anticipated, but the combined garrison of about 12,000 was smaller than other garrisons the Marines had already defeated. The military confidently asked for only a one-week delay to gather some additional troops and resources.

What the intelligence did not show was that the Japanese garrison, commanded by Colonel Kunio Nakagawa, was radically reinventing island defense tactics. The easy American victories after Guadalcanal shook many Japanese officers, including Nakagawa. Peleliu was a perfect laboratory for these new ideas.

The island is mostly limestone, and heavy annual rains have eroded complex systems of caverns and sharp outcroppings out of the landscape. Peleliu's west is composed of high, rugged hills. Korean and Taiwanese laborers were shipped out to create a complex tunnel network between the caves. Artillery and machine guns were emplaced in strategic positions, giving the defenders interlocking fields of fire over much of Peleliu and a huge number of ambush points.

American planners missed all of this, because the island's heavy tree cover disguised the chaotic terrain and the feverish Japanese preparations. They assumed the island was mostly rolling hills like others they'd encountered.

The invasion force for Peleliu and Anguar was composed of the First Marine Division and the 81st Infantry Division, backed by the Navy's Third Amphibious Force. 48,000 officers and men. They expected a cakewalk over the islands' combined six square miles - the operational plan called for five days of active combat.

The three-day bombardment of the island caused very few Japanese casualties. When the Marines went ashore, they came under fire from almost every direction. Not only were the Japanese on ideal defensive terrain, Nakagawa imposed a ban on banzai charges; Japanese counterattacks were disciplined and failed attacks ended in retreat instead of suicidal frenzies.

The First Marine Regiment took 70 percent casualties before it was rotated out of the front lines on September 23. A large part of this agony was self-inflicted; the temperature was over 100 degrees Fahrenheit and the combat troops were going through water much faster than anticipated. Some unsung genius decided to start giving the frontline troops water from cans previously used to store gasoline and not decontaminated.

The fight took a month. A battalion of Japanese reinforcements came over from Palau on rafts, although this relief force took very heavy casualties. After a month of combat, the island was declared secured, but fighting continued until the war's end and beyond. Colonel Nakagawa committed suicide on November 25, and U.S. engineers blasted the caves of Japanese holdouts shut.

Incredibly, a few survivors tunneled out in February 1945, but were taken prisoner after emerging. Other holdouts prowled the mountains until April 1947, when a Japanese admiral arrived to relieve them of duty and inform them of the war's end.

The Battle of Peleliu (and Anguar) was fought out of overconfidence, and it cost the United States almost 2,000 dead and 8,000 wounded. Japan lost over 12,000 dead in the battle. Had the U.S. military had better warning of the cost of this campaign, it might have decided to restrict operations in Palau to bombing the airfield into submission. As it stands, it could be argued that the campaign was, all in all, worth fighting - because it served as a wake-up call that forced more careful planning before the similarly brutal fights on Iwo Jima and Okinawa.

Sources: Bill Sloan's Brotherhood of Heroes: The Marines at Peleliu, 1944, Jim Moran and Gordon Rottman's Peleliu 1944: The Forgotten Corner of Hell, Kit Carter and Robert Mueller's Combat Chronology, 1941-1945.

The cost

The battle went differently because the Japanese unveiled new tactics (dig in, defend in depth, eschew suicidal charges, use the terrain as much as possible) that proved far costlier to the US forces than previous invasions. While the airfield fell quickly the rest of the island was a maze of defensive fortifications that were difficult to conquer. This led to the battle's sanguine nickname: Bloody Peleliu.

Peleliu's Strategic Worth

As to the "why invade" portion of your question, the objective was the seizure of the airfield in order to secure it prior to the invasion of the Philippines. The reality of the strategic worth of the objective has been debated ever since. It cannot be denied that after it was seized Peleliu ceased to be an airbase for the Japanese and did not contribute to their defense of the Philippines. How much this base could have contributed to that defense is more questionable, as was the utility of the airfield and the island to future US operations. However, it is often overlooked that the US learned a number of valuable--if costly--lessons regarding this new style of warfare. Future naval bombardments were much more effective, coordination between ground forces and their supporting services (land, sea, and air) improved in order to suppress enemy strongpoints, and tactics were developed to counter Japanese fortifications. None of this should minimize the horrors of the losses on Peleliu, but it does at least give a silver lining to a clouded issue.