Every war needs a casus belli, a latin expression meaning the justification for declarations of war; casus means "incident" or "case" while belli means bellic ("of war"). A casus belli might be something genuine and reasonable: a threat or an attack on an ally for example. However, a casus belli might also be an excuse created in the absense of a genuine reason: one country has something another wants or one country has some unfinished business left over from a previous war. Many of these excuses have become famous for their flimsiness. The War of Austrian Succession was started over the removal of an English Captain's ear by an overexcited Spanish customs officer. The city states of Modena and Bologna fought over a wooden bucket. The Franco-Prussian War was started over the hyped up insults towards the French ambassador contained in the Ems Dispatch. The Pastry War between France and Mexico, as well as the Chaco War between Paraguay and Bolivia, both fall into this category of wars started on the basis of a particularly tenuous casus belli.
The origins of the Pastry War of 1838-39 lie in the chaotic civil disorder that plagued the early years of the Mexican Republic, which gained independance from Spain in 1821. In 1828, the Mexican President Manuel Gomez Pedraza ejected the governor of the state of Mexico, Lorenzo de Zavala, from his office. Zavala promptly gained the support of General Antonio Lopez de Santa Anna (of Alamo fame) and rallied most of the garrison in Mexico City to his cause. After four days of fighting Zavala won, reinstated himself and installed a new president, Vicente Guerrero. The fighting damaged a lot of foreign property in the city and the subsequent riots led to further widespread destruction and looting. Though foreigners were unable to obtain compensation, Zavala's return to power eventually stabilised the domestic situation.
Ten years later, in 1838, a French pastry chef named Remontel suddenly remembered that his shop in the Tacubaya district of Mexico City had been looted by Mexican soldiers during the chaos and demanded compensation from the Mexican government. His claim was ignored so Remontel then appealed to the king of France, Louis Philippe. It just so happened that Mexico had defaulted on millions of dollars worth of loans from France, so it isn't difficult to see a connection between this default and the unfolding events. Though France had remained inactive in 1828, she now demanded 600,000 pesos in compensation for her citizen, a staggering sum considering that at the time the average wage in Mexico City was one peso a day. France then sent an ultimatum, threatening to blockade the country and seize its possessions if Remontel wasn't paid.
When the Mexican president, Anastasio Bustamante, refused to pay, France kept good on her promise and sent a fleet under Captain François Bazoche to blockade Mexico's Atlantic coast, bombard the fortress of San Juan de Ulua, and capture the major port city of Veracruz, where the French also seized most of the Mexican fleet. Mexico responded by declaring war and recalled the retired General Santa Anna to the conflict (the general had previously been disgraced by his defeat during the Texas Revolution and used his service in the Pastry War to re-enter Mexican politics as a hero). Fighting in and around Veracruz continued until the British intervened politically, brokering a peace. On 9 March 1839, France received a 600,000 dollar indemnity and both nations granted the other favoured trading status. Though France also restored the installations it had seized or destroyed, she managed to keep the Mexican fleet.
The Chaco War of 1932-35 opposed Paraguay and Bolivia, two of South America's poorest countries, in the bloodiest military conflict on that continent in the 20th century. By the late 1920s tensions were increasing between the two nations over an area of land between them known as the Gran Chaco, which, though thinly populated and arid, was thought to be rich in oil. Paraguay had lost almost half its territory to Brazil and Argentina in the War of the Triple Alliance, and wanted the Chaco simply so as to regain some status. Bolivia had lost its only ocean access (to the Pacific) in the War of the Pacific with Chile, and wanted the Chaco for access to the Atlantic via the Paraguay River, which ran through the region. Both countries were desperate and not prepared to concede without a fight.
In 1928 Paraguay brought the tension to a head by issuing a 1.50 peso stamp of the country with the Chaco included and labelled "Chaco Paraguayo". This was followed by a bolder, more provocative stamp featuring the Chaco region alone, again labelled Chaco Paraguayo. The Bolivian postal service responded, in 1931, with their own 25 centavos stamp, showing Bolivia with the Chaco included and labelled "Chaco Boliviano". Bolivia then mounted a full military assault on a Paraguayan garrison, prompting Paraguay to declare war.
Bolivia's population was three times larger than that of Paraguay. Bolivia also had lucrative mining income and a larger, better-equipped army. However, Bolivia never had more than two thirds of its army in the Chaco at any one time. In addition, landlocked Bolivia faced significant difficulty importing military equipment because of an international arms embargo imposed by the League of Nations. On the other hand, the Paraguayan military received supplies and intelligence from Argentina. The Paraguay's army also relied on guerrilla tactics that were highly effective against the more conventional Bolivian strategy, and its troops could communicate in the local Guarani language, incomprehensible to the average Bolivian soldier. Lastly, Paraguayan troops could easily be brought to the region by the Paraguay River, while Bolivian troops had to march in from 800 kilometers away.
The fighting was intense, with both sides deploying tanks and aircraft, a first in the Western hemishpere. By 1934 both Paraguay and Bolivia were on the verge of bankrupcy. On 27 November a group of Bolivian generals launched a coup d'etat which replaced the president. This political change led to a ceasefire being declared in June of 1935. A truce was finally signed in 1938, though a final treaty clearly marking the border would have to wait until 2009. Paraguay was ceded three quarters of the Chaco (oil was indeed found there in 2012) while Bolivia received its desired peice of river frontage. Paraguay celebrated the victory appropriately, by issuing a stamp.