為什麼說美國南北戰爭是一場違憲的戰爭？
先問是不是：
美國南北戰爭是一場違憲的戰爭嗎？然後問為什麼：不是南方聯邦先動的手嗎？就算南方聯邦可以合法脫離，那北方挨打還不能還手？
先說一下目前排名第一的答案的一點法理學上的錯誤。
為什麼說美國南北戰爭是一場違憲的戰爭？ - 你球藥丸的回答
各州毫無疑問是可以脫離聯邦的，因為沒有任何法律規定不允許脫離。即使打官司到最高法院，最高法院也沒法判。如果按照成文法，沒有法律規定；而如果按照習慣法，美國脫離英國怎麼算？
好了或者我們不妨來個假設，即使南北戰爭後也沒有任何法律規定不允許退出聯邦，那麼如果得克薩斯州今年公投獨立了，請問美國政府打算制止嗎？當然了，戰後最高法院在Texas v. White這個案子里重新解釋了獨立問題，最高法院的解釋是獨立是不合法的。但問題是，那是戰爭後，戰爭前並沒有這個判例。而且即使戰後有了這個判例，今天這個時代，請問美國政府真敢執行這個判例嗎？
第一段最後一個問題很簡單，美國脫離英國當然就是違憲的（unconstitutional/illegal）。也就是說，這是一場革命。借用凱爾森在《法與國家的一般理論》中對革命的說法：
從法學觀點來看， 革命的決定性標準就是現行秩序已由新秩序在以前一個秩序本身所不曾料到的方式下加以推翻和代替。
實際上《獨立宣言》也就這麼說的：
that they are Absolved from all Allegiance to the British Crown, and that all political connection between them and the State of Great Britain
按照當時對英國憲法的定義，英國憲法是Mixed Constitution，英國憲法就是King-in-Parliament。美國革命的行為就是廢除King-in-Parliament的Mixed Constitution，並制定新憲法。
第二個問題，需要分清楚的是，戰後最高法院的行為是立法行為亦或是法律解釋？如果是立法行為，那麼除非它明確表達，否則它是沒有溯及既往的能力的。
如果是法律解釋，那麼它就是溯及既往的。因為很簡單，法律解釋的效力是附著於原始條文的而非另立新法。所以戰爭前後與否，沒有什麼意義。- 打個比方：某條刑法規定某項罪行，人們對此有爭議。某人因為這項條文而被控告。法院在判決中對此做出澄清，並判處此人刑罰。法院的這個行為並不違反刑法不溯及既往的原則，因為本質上這是對法律的解釋，某人因為對法律的錯誤理解而違法。
美國聯邦最高法院的法律解釋在法律體系上具有最高效力，那麼，無非就是戰爭前南方人認為有退出權對憲法的理解錯了，後來最高法院才釐清而已。
至於「請問美國政府真敢執行這個判例嗎」這個問題，則對法理學上兩個相近的概念沒有弄明白：效力（validity）和實效（efficacy）。法理學和政治哲學關心前者，法律社會學和政治科學關心後者。以下是私貨：南方脫離聯邦的行為究竟是否是違憲？或者說是否是革命行為？當然就是，個人覺得沒什麼好說的。憲法一詞的原意和本質是政體（polity）。除非像蘇聯憲法那樣直接規定退出權及其程序，否則必定是需要修憲程序的。與其大家按照自己的理解去解釋，倒不如去讀讀各州的脫離宣言，看一下他們自己對此的解釋是什麼。以南卡為例（如果我沒記錯的話，南卡的脫離宣言是最早通過的）。
"Declaration of Immediate Causes Which May Induce and Justify the Secession of South Carolina from the Federal Union," 24 December 1860。根本論點就是《憲法》條約論：and we hold further, that the mode of its formation subjects it to a third fundatmental principle, namely: the law of compact.
該宣言的其他論點，比如《獨立宣言》的sovereign state論、第十修正案保留權力論，都必須和這個論點結合起來，不然沒有任何意義。即，聯邦憲法是各州（states）之間的聯合協議，而聯邦政府和廢奴各州的廢奴行為侵害了南方各州的財產權和主權從而違背了協議。也只有在憲法條約論的基礎上，才能有單方面的退出權，否則即使有退出權也不是單方面的。
The Guaranties of the Constitution will then no longer exist; the equal rights of the States will be lost. The slaveholding States will no longer have the power of selfgovernment, or self-protection, and the Federal Government will have become their enemy.
請注意引文的guaranties一詞，這也是建立在《憲法》條約論的基礎上的，將憲法理解為一旦一方違約就可以撕毀的條約。南卡據此認為，聯邦已經不存在了。
而如果是法律的話，則不同於條約——即使法律治下的某些人（法人）違法，也不代表法律失效。have solemnly declared that the Union heretofore existing between this State and the other States of North America, is dissolved,
heretofore一詞的意思是「在此之前」、before now。從這句話而言，其實南方並不是用的「退出權」，而是說，聯邦已經解散了。
同時，請注意第十修正案的措辭：The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
the people而非the peoples，這個措辭和憲法序言開頭一樣：We the people of United States,...。換句話說，確定憲法的是「人民」整體，而非各州（states）或各州人民（peoples）之間的協議。某一州要退出憲法，也要所有美國人民決定。也正是如此，We the People這三個字在美國憲法解釋上具有重要意義。回顧一下美國憲法的序言，請注意我標粗的字詞。
We the people of the United States, in order to form a more perfect Union, establish justice, insure domestic tranquility, provide for the common defense, promote the general welfare, and secure the blessings of liberty to ourselves and our posterity, do ordain and establish this Constitution for the United States of America.
其中，橫線是從句，拿掉從句則句子是這樣的：「We the people of the United States do ordain and establish this Constitution for the United States of America」。這就很好讀了。
其實，在《獨立宣言》中的說法就是混合的了。一方面，正如南卡的宣言中所說，獨立宣言規定了各州的sovereignty。另一方面，獨立宣言又宣稱是one people/a free people。大家可以對照另一份文件，聯合國憲章序言的措辭就是用的peoples。We the peoples of the United Nations Determined:
類似的，未能生效的歐盟憲法條約和替代它的里斯本條約，在序言部分的措辭都是「the peoples of
Europe」。
先說是不是：南方各州當然違憲，北方無論以平叛還是吞併的理由當然符合憲法。所以南北戰爭並不違憲。
No state shall enter into any treaty, alliance, or confederation; grant
letters of marque and reprisal; coin money; emit bills of credit; make anything but
gold and silver coin a tender in payment of debts; pass any bill of attainder, ex post
facto law, or law impairing the obligation of contracts, or grant any title of nobility.
No state shall, without the consent of the Congress, lay any imposts or duties on
imports or exports, except what may be absolutely necessary for executing it s
inspection laws: and the net produce of all duties and imposts, laid by any state on
imports or exports, shall be for the use of the treasury of the United States; and all
such laws shall be subject to the revision and control of the Congress.
No state shall, without the consent of Congress, lay any duty of tonnage, keep
troops, or ships of war in time of peace, enter into any agreement or compact with
another state, or with a foreign power, or engage in war, unless actually invaded, or in
such imminent danger as will not admit of delay.
南方各州組建新的聯盟，越過國會擅自開始外交以及宣戰，依然是違憲的。
同樣的，憲法第十修正案
The powers not delegated to the United States by theConstitution, nor prohibited by it to the States, are reserved to the
States respectively, or to the people.憲法沒有授予聯邦政府的權力依然屬於州政府或者人民。這就是法無授權即禁止最好註解。憲法沒有規定如何脫離聯邦，所以這個權力依然歸公民所有。為啥不是州政府？很簡單，憲法序言，組建聯邦的權力來自於公民，而非州政府。綜上所述，南方各州既無權脫離聯邦政府，也無權組建新的聯盟，更無權自行進行外交發行貨幣組織軍隊參加戰爭。無論從哪一點來說，都是違憲的。
至於南北戰爭不違憲的原因，可以參見 @你球藥丸的答案。
可以這麼看，根據美帝法律，南方脫離不違憲，所以美國變成了兩個國家。
但因為美國憲法沒有規定美國不能發動戰爭吞併他國，所以美國找了個借口把美聯錘了，重新統一了美國。但是宣傳不能這麼宣傳，反正美帝自己肯定不能說北方打南方是侵略戰爭。美利堅聯盟國：為什麼會是這樣呢……明明……明明是我先的……獨立也好，開戰也好……為什麼要說北方違憲呢？美利堅合眾國：你們知乎有一個好，黑起美國來你們跑得比誰都快，但是你們問的這個問題啊……
這個問題是區分你是不是極右翼的分界標誌。
我確實也認為南北戰爭是違憲的，是一場南方發動的試圖用違憲手段脫離聯邦的戰爭。
轉一下 @King Kwan 和 @talich 的答案：
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作者：King Kwan
鏈接：林肯發動南北戰爭是否屬於違憲? - King Kwan 的回答
來源：知乎
著作權歸作者所有，轉載請聯繫作者獲得授權。
TLDR：林肯一沒挑起內戰，二沒違反憲法。南方叛亂州為了維護奴隸制分裂國家才是最嚴重的違憲行為。
哈哈哈哈哈哈哈哈哈答主要嘲諷了：答主不是針對某個lost causer，答主是說在座的lost causers都是垃圾。
lost causer不愧為美國的果粉，從智商到臉皮全方位向中國果粉看齊。一百五十多年過去了，有幾個美國人還記得……美國人確實歷史水平堪憂，然而這種鬼話拿來唬傻小子都唬不了。南方退出聯邦，美其名曰「維護州權」，州權個燈籠，不就是想家裡養奴隸嗎？還意淫什麼憲法賦予的權利，也是病的不輕。南方強行分裂後要求薩姆特堡守軍投降，被拒後主動開火引發內戰，到頭來居然算到林肯身上？答主好歹也是AP美國歷史考過5分的人，花式黑林肯的教材見得多了，就是沒見過批判林肯發動內戰的。題主可否詳細告知您看的是哪個州哪個出版社出的哪個無賴編的哪本教材？
說什麼林肯維持聯邦是違憲，狗奴隸主也配提憲法？憲法開頭怎麼說的？
We the People of the United States,
in Order to form a more perfect Union, establish Justice,insure
domestic Tranquility, provide for the common defence,promote the
general Welfare, and secure the Blessings of Liberty to ourselves and
our Posterity, do ordain and establish this Constitution for the United
States of America.
拿憲法說事簡直不知羞恥。脫離聯邦不就是最嚴重的違憲行為嗎？建立完正的聯盟、匡扶正義、保障內部和諧、共同捍衛國家安全、促進公共福利，南方的叛亂州做到哪一點了？
The powers not delegated to the United States by the
Constitution, nor prohibited by it to the States, are reserved to the
States respectively, or to the people.
這是lost causer們最愛的第十修正案，答主就問問哪句明確說了州可以退出聯邦？如果說沒明確說就是默許的話，憲法也沒允許或禁止聯邦政府維持聯邦，批判林肯的底氣又是哪來的？
面對南方的分裂，林肯是這麼說的
What is now combatted, is the position that secession is consistent with
the Constitution—is lawful, and peaceful. It is not contended that
there is any express law for it; and nothing should ever be implied as
law, which leads to unjust, or absurd consequences. The nation
purchased, with money, the countries out of which several of these
States were formed. Is it just that they shall go off without leave, and
without refunding? The nation paid very large sums, (in the aggregate, I
believe, nearly a hundred millions) to relieve Florida of the
aboriginal tribes. Is it just that she shall now be off without consent,
or without making any return? The nation is now in debt for money
applied to the benefit of these so-called seceding States, in common
with the rest. Is it just, either that creditors shall go unpaid, or the
remaining States pay the whole? A part of the present national debt was
contracted to pay the old debts of Texas. Is it just that she shall
leave, and pay no part of this herself?
幾個叛亂州的土地都是聯邦政府借錢買的，德克薩斯州尤甚。叛亂州把債一甩拍屁股走人，讓聯邦政府接著還錢？無恥也要有個限度好吧。
林肯並沒有拿奴隸制說事，本來北方打內戰也不是為了黑奴打的。實際上直到後來北軍缺人的時候他才搞出來《解放黑人奴隸宣言》，想挖一把南軍的牆角。然而南方這些道貌岸然的奴隸主老爺，可是時刻懸著一顆心吶。別說答主聖母心，美國的奴隸制、奴隸貿易就是人類歷史上黑暗血腥的一頁，誰敢否認？不管lost causer怎麼百般抵賴，南方為了維護奴隸制分裂國家發動內戰的事實都是洗刷不清的。脫離聯邦的理由么，奴隸主們說得清清楚楚，lost causer們不要像果粉一樣裝看不見。
喬治亞州
The people of Georgia having dissolved their political connection with
the Government of the United States of America, present to their
confederates and the world the causes which have led to the separation.
For the last ten years we have had numerous and serious causes of
complaint against our non-slave-holding confederate States with reference to the subject of African slavery.
密西西比州
Our position is thoroughly identified with the institution of slavery — the greatest material interest of the world. Its labor supplies the
product which constitutes by far the largest and most important portions
of commerce of the earth. These products are peculiar to the climate
verging on the tropical regions, and by an imperious law of nature, none
but the black race can bear exposure to the tropical sun.
南卡羅萊納州
We affirm that these ends for which this Government was instituted have
been defeated, and the Government itself has been made destructive of
them by the action of the non-slaveholding States. Those States have
assume the right of deciding upon the propriety of our domestic
institutions; and have denied the rights of property established in fifteen of the States and recognized by the Constitution; they have denounced as sinful the institution of slavery;
they have permitted open establishment among them of societies, whose
avowed object is to disturb the peace and to eloign the property of the
citizens of other States. They have encouraged and assisted thousands of our slaves to leave their homes; and those who remain, have been incited by emissaries, books and pictures to servile insurrection.
德克薩斯州
Texas abandoned her separate national existence and consented to become
…all white men are and of right ought to be entitled to equal civil
one of the Confederated Union... She was received into the
confederacy...as a commonwealth holding, maintaining and protecting
the institution known as negro slavery — the servitude of the African to
the white race within her limits — a relation that had existed from
the first settlement of her wilderness by the white race, and which her
people intended should exist in all future time.
In all the non-slave-holding States… the people have formed
themselves into a great sectional party… based upon an unnatural feeling
of hostility to these Southern States and their beneficent and
patriarchal system of African slavery, proclaiming the debasing doctrine
of equality of all men, irrespective of race or color — a doctrine at
war with nature, in opposition to the experience of mankind, and in
violation of the plainest revelations of Divine Law. They demand the
abolition of negro slavery throughout the confederacy, the recognition
of political equality between the white and negro races, and avow their
determination to press on their crusade against us, so long as a negro
slave remains in these States
and political rights; that the servitude of the African race, as
existing in these States, is mutually beneficial to both bond and free,
and is abundantly authorized and justified by the experience of mankind,
and the revealed will of the Almighty Creator, as recognized by all
Christian nations…
敬愛的大總統戴維斯，千萬lost causer的精神領袖，千古完人戴公說得尤其好
Our new government is founded upon exactly the opposite idea; its
foundations are laid, its corner-stone rests, upon the great truth "that the negro is not equal to the white man; that slavery subordination to the superior race is his natural and normal condition. This, our new government, is the first, in the history of the world, based upon this great physical, philosophical, and moral truth.
以壓迫和剝削為榮，於黑奴的血淚與屍骨之上聚攏財富，拿崇尚公義的憲法做擋箭牌——答主從未見過如此厚顏無恥之人。
歡迎精神南方人前來撕逼。
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@talich 的答案：
美國南北戰爭的真正原因是什麼？ - talich 的回答______________________________________________________________________
兩面立場都給些材料，這裡補充一個對南北戰爭的法醫系（劉仲敬）意識形態解釋：
阿姨學註疏：南北戰爭前後的憲法鬥爭誰擁有更強的暴力，誰就有權解釋和執行法律。
有些個答案。。。額受不了了說什麼林肯維持聯邦是違憲，狗奴隸主也配提憲法？憲法開頭怎麼說的？
We the People of the United States,
in Order to form a more perfect Union, establish Justice,insure
domestic Tranquility, provide for the common defence,promote the
general Welfare, and secure the Blessings of Liberty to ourselves and
our Posterity, do ordain and establish this Constitution for the United
States of America.
拿憲法說事簡直不知羞恥。脫離聯邦不就是最嚴重的違憲行為嗎？建立完正的聯盟、匡扶正義、保障內部和諧、共同捍衛國家安全、促進公共福利，南方的叛亂州做到哪一點了？
首先，脫離聯邦前，南方各州哪裡違憲了呢？然後，脫離聯邦怎麼就變成了違憲行為了呢？憲法里有這個條款？如果沒有的話，人民有權脫離聯邦政府，這叫「法無禁止即自由」The powers not delegated to the United States by the
Constitution, nor prohibited by it to the States, are reserved to the
States respectively, or to the people.
這是lost causer們最愛的第十修正案，答主就問問哪句明確說了州可以退出聯邦？如果說沒明確說就是默許的話，憲法也沒允許或禁止聯邦政府維持聯邦，批判林肯的底氣又是哪來的？
憲法沒有授權聯邦政府的權力，聯邦政府當然不擁有，這叫「法無授權即禁止」。是
邦聯國那是在自己的地盤上動手的，又沒侵略北方就算南方有獨立的權力，北方也不過就當又打了一次墨西哥戰爭，搶了加利福尼亞而已。
這事就是美國人非要給自己留點臉面所以攪混水而已。
南方各州脫離聯邦沒有違憲
但是一旦脫離，美利堅合眾國的所有法律就保護不了他們了啊
1860年，侵略一個主權國家又不是啥大事
於是美利堅合眾國舉兵侵略美利堅聯盟國，滅國吞併。
這事就算完了唄。
美國憲法禁止美國侵略另一個主權國家了？並沒有，近的有阿富汗、伊拉克、利比亞；遠的有墨西哥、夏威夷、海地。
所以古有林肯歪解憲法悍然入侵美利堅聯邦，今有拉姆斯菲爾德高舉洗衣粉為名入侵伊拉克。侵略戰爭雖然不是正義戰爭，被其他國家的吃瓜群眾所鄙視，但是在發動戰爭的美利堅合眾國角度，這哪裡違憲了？南方合法脫離 北方也可以合法地把南方當墨西哥打
有關Lost Cause：
都忘了高院的那句話？
我們並非因為權威而強權，而是因為強權而權威。
美國違憲的事情太多了。 蘇特兒發現加州金礦，理論上整個加州的金礦都是他的... 結果呢？ 法不責眾。槍杆子里出政權 拿衣服
排名第一的答案是反對上一個排名第一的答案的……試著反對一下吧。
關於法律解釋的效力那一段，法律解釋的效力是溯及既往的。但是對於新法律解釋出台前已經確定的裁決，除非新法律解釋是有利於被告的，否則應該無效吧？
據此，依據事後的最高法院解釋裁決之前的南方人違憲恐怕不妥？
另，假如南方脫離聯邦是合憲的，那麼為反對南方脫離聯邦發動戰爭的北方政府就是違憲的。自其發動戰爭起政府即失去合法性，政權應當解體。
由此後來的聯邦政府應該從法源上被否定，除非美國人再簽一遍獨立宣言。
所以南方脫離聯邦是否合憲更不應該由事後的最高法院裁定。
換句話說有可能真正的美國人已經沒了……現在存在的只不過是偽法統治下的北美人民聯合體……
據此……
蘇聯可能是被北美人民自發組織起來的聯合體打敗的……
構成世界政治秩序的雅爾塔宣言啊啥啥啥的通通無效。
腦洞是不是大了點法無禁止即為允許，當時美國憲法又沒禁止各州自行脫離合眾國。
個人在看了一些文章或電視節目後覺得美國南北戰爭是違憲的。
大家可以觀看高曉松的曉說有這一期的節目。
首先在美國憲法中確實有保護蓄奴的權利，而林肯要廢除蓄奴，北方人民覺得侵犯了憲法給予的權利，所以決定用戰爭來進行反抗。林肯答應南方奴隸在戰爭勝利後給予他們自由。南方的軍隊大部分士兵都是黑人。
以上僅代表個人觀點。推薦閱讀：
※如何評價查爾斯頓槍擊案以後美國各地去除邦聯遺迹的風潮？
※南北戰爭在美國歷史上的意義是什麼？
※林肯一生除廢除黑奴制，領導取得南北戰爭勝利外還有什麼貢獻或是成就？