War may begin with a war of words and with wrong assumptions/mistakes… between opposing countries. And things may escalate quickly and become violent… War may end with many fatalities and much destruction.

Wars can start on trivial issues and escalate quickly depending on the level of acrimony between the hostile nations. There are instances when a war started with the killing of one person. The best example of such a war is the First World War which started with the assassination of Archduke Ferdinand,heir to the Austro-Hungarian Empire who was killed for political gains but it quickly escalated into a war which engulfed the entire continent of Europe 

A.J.P Taylor, the British historian, believed the First World War occurred by accident. Several armies became organised, a few to prearranged plans, and the mobility of rail travel prevented the process from being stopped.

Historical evidence reflects that wars most frequently begin because of a certain dispute between countries, which is often very childish in nature. When the power of reasoning and logic is diminished , the ground for the outbreak of a war becomes ready.On this basis, I agree with Margaret Atwood who says, "War is what happens when language fails.” As for the second part of the question, after the parties involved in the conflict see the futility of the war and the enormity of the human loss thereof , they usually seek a way to bring it to an end.

interference of big powers in other countries are main cause of wars, other factor is economy. ammunition dealers also support wars among enemies, based on race, beliefs etc

War, a destroyer and an impediment to progress always start with disagreement between two countries motivated by greed, hatred, selfishness, escalated by action and inaction of the authorities involved, ammunition's dealer and some politicians. The end is always what a progressive community will wish for: economy setback and instability, spread of diseases, displacement of citizens and so on.

Main Causes of War: Economic Gain, Territorial Gain, Religion, Nationalism, Revenge, Civil War, Revolutionary War, and Defensive War.

The opposing countries have to be convinced about the aftermaths of war. The war can only be avoided by mutual talks and compromising on a point.

Coming Soon Unfortunately: Global Nuclear War

China’s Xi tells military to prepare for war as US Navy warns of high seas encounters,

https://www.rt.com/news/442625-china-prepares-war-us/

"Backing Into World War III:

America must check the assertive, rising powers of Russia and China before it’s too late. Accepting spheres of influence is a recipe for disaster.",

https://foreignpolicy.com/2017/02/06/backing-into-world-war-iii-russia-china-trump-obama/

"Mikhail Gorbachev warns of new Cold War with US-Russia arms race 'in full swing'",

https://www.independent.co.uk/news/world/politics/mikhail-gorbachev-new-cold-war-us-russia-arms-race-full-swing-soviet-premier-putin-trump-north-korea-a7688321.html

"Nuclear war: Hypothetical scenario & Russia's strike options",

https://www.rt.com/op-ed/442774-nuclear-inf-war-scenario/

"Russian Bear bomber gate crashes NATO’s largest drill since Cold War",

https://www.rt.com/news/443049-russian-plane-nato-drills-bear/

"Senators propose change of doctrine so Russia could respond with nukes to any ‘strategic strike’",

https://www.rt.com/russia/444573-russian-nuclear-doctrine-amendment/

"The nuclear threat is rising. Europe cannot just stand and watch...",

https://www.theguardian.com/commentisfree/2018/nov/26/nuclear-threat-europe-putin-trump-terrorist

"US, Europe & NATO risk all-out war by backing unhinged Kiev regime",

https://www.rt.com/op-ed/444997-nato-war-ukraine-azov-ships/