The Chaco War took place in the Northern Chaco Boreal in South America. The war was fought between Paraguay and Bolivia. It was the largest inter state war in the Americas in the 20th century.

The exact cause of the war has always been unclear. However the border between the two was poorly defined and there had been a number of previous confrontations over the years. The territory in which the war took place was officially considered as a part of Paraguay.

Paraguay had over the years built a number of small forts across it to reinforce its claim. With the Chaco being a harsh environment many of the forts were built around oasis. This would initially be of advantage but then of great harm to the Bolivian offensive.

Further to the unclear border there was also a growing belief that the area held oil. The Chaco itself is a dry dusty place which does no encourage settlement but if oil had been found its value would have increased vastly.

A little further to the north in Bolivia oil had been found increasing the belief that the Chaco also concealed large deposits. Paraguay and Bolivia were both supported by rival oil companies. However quite how much their actions fermented the conflict is debatable.

One further benefit Bolivia saw to be gained from a war was access to the sea. A generation earlier following the loss of the War of the Pacific it had become landlocked. Gaining control of the Chaco would give Bolivia access the the Rio Paraguay and from there a maritime route to the Rio Plate and the Atlantic.

Although both sides employed large numbers of horses the terrain in the Chaco was not suitable for the deployment of cavalry. This made the Chaco War very much a mechanized one. Trucks, tanks and planes were used by both sides and the Paraguayans were able to bring troops towards the front by train.

The Chaco War saw the first large scale aerial combats in the Americas.

Bolivia started the war with large advantages in both troop and vehicle numbers. These were though nullified by the conditions in the Chaco. Most of their troops came from the high Andes and had no expeirience of hot, dry desert lowlands. Furthermore the vehicles did not cope well with the sand that constantly blew around.

Tactics also favoured the Paraguayans. The Bolivians attempted to fight a traditional war through taking possession of strong points and digging in.

The Paraguayans however under the leadership of Jose Felix Estigarribia fought a far more mobile war. Theirs was one of forced marches and encirclements. Time and again cutting off and defeating stationery Bolivian units.

The first actions of the war occurred on 15th June 1932. On that date Bolivian forces crossed into Paraguayan territory. They attacked and burnt down one Paraguayan fort before moving on to capture three more.

Paraguay responded to this invasion with a general troop mobilization. The army then moved north into the Chaco and after a month long siege recaptured the fort at Boqueron. An event still remembered today as a national holiday.

The Bolivian response to their initial defeat came in January 1933. The army once more advanced into the Chaco. The initial attempts to take forts all failed. However the campaign continued and a number of forts were captured before the advance was stopped and then pushed back by the Paraguayan forces.

The captured forts were heavily fortified by the Bolivians over the next few months. Then in September the Paraguayans lead by Estigarribia began a campaign to dislodge them. Through a brilliant campaign of forced marches through the trackless wilderness and encirclement after encirclement the forts were all retaken and their Bolivian defenders evicted.

In their rush to escape the Bolivians left behind vast amounts of supplies. Large numbers of tanks and other vehicles along with thousands of guns fell into Paraguayan hands. The majority was German made and of a far better quality than thee old French arms the Paraguayans had previously been reliant upon.

A truce was declared in December 1933. This held until January 1934 allowing the Bolivian forces to pull back to the border.

Once the truce expired the Paraguayans began pushing against the Bolivian defenses searching a way through their lines. The Bolivians had some success in holding back the Paraguayans but by December 1934 gaps had opened in the lines allowing Paraguay to cross the Rio Pilcomayo into Bolivia.

Once inside Bolivia a portion of the Paraguayan army prepared to lay siege to the fortified Bolivian town of Villa Montes. Whilst that happened the remainder of the Paraguayan army advanced swiftly into Bolivia and towards their oil fields.

When a truce was declared on 12th June 1935 the Paraguayan forces were just a few miles short of the oil fields. By them the Bolivians had been comprehensibly defeated. Their third major defeat in less than 100 years.

After the war the ceasefire became a permanent peace in 1938 when the border was finally officially designated. However even then the war was not completely confined to history. The final documents relating to the war were not signed until 28th April 2009.

However since 1935 Paraguay, Bolivia and the Chaco have remained at peace. The large oil deposits proposed to be lying under the sand have still not though been discovered.