what neighboring countries became enemies leading to war?

The Korean peninsula was divided along ideological lines after the Second World War. Years later, the Korean War was waged from 1950 to 1953 - the communists in the North attempted to conquer the democratic South and the two countries exist in a state of war to this day.

The former Balkan nation of Yugoslavia split into several independent states in the early 1990's - Serbia, Croatia, Slovenia,Bosnia, Montenegro, and Macedonia. Kosovo gained its independence years later but its sovereignty is still in dispute in the eyes of several countries. Wars raged between Serbs and Croatians and Bosniaks, Macedonians, Montenegrins and Kosovars for years and many atrocities were committed by all factions.

Eritrea and Ethiopia fought for 30 years from the early 1960's to the early 1990's before Ethiopia finally recognized the breakaway republic as an independent nation.

Iran and Iraq fought for over 8 years and some sources estimate the number of casualties as being close to one million. Neither side gained much of a strategic or territorial advantage before hostilities came to an end.

South Sudanese rebels were engaged in a bloody conflict with the Sudanese government of Khartoum for years. There is evidence to support the claim that the Sudanese engaged in ethnic cleansing against the Black Sub-Saharan natives of South Sudan. The newly independent nation of South Sudan came into existence 5 years ago and there are reports that South Sudanese militia have been clashing with Ethiopian citizens along the shared border.

Somalia has not had a functional federal government in over two decades and various tribal leaders and militia groups have been fighting for control of various parts of the country.

Many African nations have engaged in civil wars in the past few decades, with the Rwandan conflict being the prime example. Other wars have been fought in Liberia, The Congo, Angola, and Namibia. Tanzanian troops invaded Uganda to depose Idi Amin.

Vietnamese troops invaded Cambodia to overthrow the Khmer Rouge government which was responsible for the deaths of over 3 million Cambodian citizens. When I was in Cambodia several years ago, Cambodian troops traded fire with their Thai counterparts over the status of a disputed temple that sits on the border.

Russian troops clashed with the Georgian military following the secession of South Ossetia. Ukrainian troops clashed with Russian troops over Crimea and both sides claimed that the other had perpetrated incursions onto its territory.

There were many conflicts in many of the former Soviet Republics in Central Asia in the aftermath of the Soviet Union's disintegration. Kyrgyzstan saw heavy fighting and there were reports of ethnic cleansing.

India and Pakistan have had an ongoing territorial dispute over the Kashmir region and many people have died in the fighting.

Muslims have targeted Christians all over Indonesia and things escalated to barbarity in East Timor before the territory was granted independence.

The Colombian government holds only nominal de facto control over a third of Colombian territory where the FARC and various cartels hold dominion. Clashes are constant and the loyalties of the locals are subject to change from one region to another. The conflict has spread to Venezuela on several occasions.

Basque separatists waged a terror campaign across Spain for many years and during the height of the attacks many innocent civilians were killed.

The status of Western Sahara has been in question for decades with Morocco facing off against various rebel groups vying for control of the territory.

Indonesia forcibly annexed the western half of Papua New Guinea to create the state of Irian Jaya.

The Sinhalese and the Tamil of Sri Lanka have been at war for decades over the political status of the island.

Greek and Turk Cypriots have been fighting for years as well.

In the Western world, the Troubles in Northern Ireland would probably be the most notorious example of neighbor against neighbor. Thousands died in terror attacks and retaliatory attacks over the years.

There are far, far too many instances to list here but these are the ones that I could think of off the top of my head.

Iran and Iraq, the break up of Yugolslava... there are lots of examples.