Guatemala never owned Belize:
The Treaty of Tordesillas (Tratado de Tordesilhas or Tratado de Tordesillas) signed at Tordesillas on June 7, 1494, and authenticated at Setúbal, Portugal, divided the newly discovered lands outside Europe between Portugal and the Crown of Castile, along a meridian 370 leagues west of the Cape Verde islands, off the west coast of Africa. This line of demarcation was about halfway between the Cape Verde islands (already Portuguese) and the islands entered by Christopher Columbus on his first voyage (claimed for Castile and León), named in the treaty as Cipangu and Antilia (Cuba and Hispaniola).
The lands to the east would belong to Portugal and the lands to the west to Castile. The treaty was signed by Spain, 2 July 1494 and by Portugal, 5 September 1494. The other side of the world would be divided a few decades later by the Treaty of Zaragoza or Saragossa, signed on 22 April 1529, which specified the antimeridian to the line of demarcation specified in the Treaty of Tordesillas. Originals of both treaties are kept at the Archivo General de Indias in Spain and at the Arquivo Nacional da Torre do Tombo in Portugal.
On 4 May 1493 the Pope Alexander VI (Rodrigo Borgia), a Aragonese from Valencia by birth, decreed in the bull Inter caetera that all lands west and south of a pole-to-pole line 100 leagues west and south of any of the islands of the Azores or the Cape Verde Islands should belong to Castile, although territory under Catholic rule as of Christmas 1492 would remain untouched.
Now back to Belize:
Spain sent expeditions to Guatemala and Honduras, and the conquest of Yucatán began in 1527. Though the Maya offered stiff resistance to Spanish "pacification", diseases contracted from the Spanish devastated the indigenous population and weakened its ability to resist conquest. In the 17th century, Spanish missionaries established churches in Maya settlements with the intention of converting and controlling these people. After the remaining Mayas abandoned their cities like Lubaantun and Lamanai in order to escape Christian conversion, the Spanish abandoned these areas because they were deemed worthless.
Northern European powers were increasingly attracted to the region by the potential for trade and settlement. These powers resorted to smuggling, piracy, and war. In the 17th century, the Dutch, English, and French encroached on Spain's New World possessions. The reefs that were difficult to navigate were ideal for hiding for Spanish warships and thus the Belize settlement was born, soon to harvest logwood on it's shores.
The rest is history:
The Treaty of London between James I of England and Philip III of Spain 1604, the Godolphin Treaty of 1670, in 1691 Guatemalan Archives "Escribania de Camaa 339" confirmed "Los Ingles" near to Indian villages by Lago de Isabel which they were forcefully depopulating, the treaty of Utrecht 1713, the treaty of Paris of 1763, Burnaby's Code in 1765, the treaty of Versailles 1783, the Battle of St. George's Caye 1798, the treaty of Amiens 1802.
The Baymen settlement was now an independant and self governing settlement of mainly British subjects (for certain purposes, in possession and under protection of his majesty, but not within the territory and dominion of his majesty - act of parliament 1817). 1821 Independence of the Empire of Mexico, including Guatemala, 1823 the United Provences of Central America, 1824 the Central American Federation, 1840 official independence of Guatemala (the five provinces of Central America).
Guatemala claimed the inheritance from Spain's old territories, but this was not possible as Mexico was the first independant nation in 1821. Mexico and Great Britain signed a boarder treaty in 1893 certifying the Sarstoon River on the south and the Rio Hondo on the north as legal boundaries of British Honduras. In 1859 Britain and Guatemala signed a boarder treaty, referring to the boundaries that existed before 1850, thus acknowledging British Honduras or Belize was NEVER owned by Guatemala. Part of this agreement for both parties was to "Use their best efforts by taking adequate means for establishing te easiest communication (cart road, rivers or both), between the fittest place on the Atlantic coast near the settlement of Belize and the Capital of Guatemala."
This is where the disagreement came, both agreed that Britain would provide technical input and that Guatemala would supply all the material needed, but where Charles Lennox Wyke thought they would share labour costs, Don Pedro de Aycinena was convinced that Britain would pay for labour in full. To resolve this problem, both parties agreed in 1863 that Britain would pay 50,000 Pounds Sterling to cover it's contribution and both would ratify this agreement within six months, which Britain did. Guatemala ratified it with "clarifications" in 1865 which Britain rejected.
1850 Clayton Bulwer Treaty, 1856 Dallas-Clarendon treaty, 1862 Colony of British Honduras 1871 British Honduras becomes Crown Colony status, 1931 cement columns were set instead of the pile stones (placed in 1859) at several locations and diplomatic notes were exchanged certifying the boundary, 1933 Britain and Guatemala certify the accuracy of the survey, 1973 British Honduras became Belize, 1981 Belizean Independence, 1992 Guatemala’s president formally recognized Belize’s independence.
I think that is it...