These are some important doctrines related to International Law, which would help law students and scholars getting a closer and a clearer view into the judgments and enforcement of international law.
International law is a system of treaties and agreements between nations that govern how nations interact with other nations, citizens of other nations, and businesses of other nations. These include standards of international behaviour, the laws of the sea, economic law, diplomatic law, environmental law, human rights law, etc.
Since most international law is governed by treaties, it’s usually up to the individual nations to enforce the law. However, there are a few international organizations that enforce certain treaties. The most notable example is the United Nations, which has 193 member states, including India.
General Doctrines related to International Law
1. Act of State
The Act of State doctrine is applicable to only those acts which are carried out by a government official or body. In deciding whether this doctrine is to be applied or not, the courts consider whether the government official was acting in their public capacity or not.
The state doctrine holds that, in the absence of an international treaty, the courts of other states cannot question the validity or legality of the acts of the state of another sovereign state or of its agents. If such questioning is thought necessary, it is to be confined to diplomatic channels.[i]
Case: Buttes Gas & Oil Co. v. Hammer and Others.
In a defamation action, issues arose as to two conflicting oil concessions which neighbouring states in the Arabian Gulf had granted over their territorial and offshore waters. The foreign relations of the United Kingdom and Iran were also involved in the dispute. The authorities concerning acts of state were reviewed for the purpose of a submission by the defendants that the action raised issues which were non-justifiable in English courts and should therefore be stayed. The motives of governments are not justifiable and courts should refrain from adjudicating upon them. The House considered an action by an officer of the Crown taken outside this country. 
Held: The principle of non-justifiability was applicable. The said doctrine was not applied in this case.
2. Doctrine of Imputability
This doctrine means internationally illegal acts or omissions contributing to the damage to foreign property, and caused in some way by organs of the state, are attributable to the state and therefore incur state’s responsibility.
Thus, there must have been state participation in the act before there can be state responsibility for it. Example – soldiers that are recruited illegally in other countries and damage properties of that nation are held liable.
Case: Thomas H. Youmans (U.S.A.) v. United Mexican States
Mexican military forces instead of protecting the American citizens which was attacked by a mob started an open firing which led to the killings of those citizens by the mob and the Mexican military.
Claim for damages of $50,000.00 was made by the United States of America against the United Mexican States and the claim was allowed.
3. Pacta Sunt Servanda
Latin formula pacta sunt servanda (agreements must be kept) is arguably the oldest doctrine of international law. Without such a rule, no international agreement would be binding or enforceable.
The principle can be traced back to religious origins. In the Koran, it has been stated that “Be true to the obligations which you undertake”. This religious principle in the Middle East soon manifested in commerce and ruled commercial contracts and transactions.
The Romans also respected this principle and was an extremely important part of their judicial workings. It even had a great role to play in Christianity.
During the renaissance, this principle was established in the theories and soon became an important part of International law.
Nuclear Test Case, Australia v. France
The Nuclear Test case that was between Australia and France is still a great source of anger and agitation between south pacific nations of Australia and New Zealand as a result of environmental vandalism by the republic of France in Australia.
From the 1960s, the conducting tests of Nuclear weapons began in the South Pacific. From 1966 to the early 1970s, this included atmospheric testing. Australia and New Zealand argued that such practices resulted in radioactive particles spreading throughout the world. In order to stop the testing, they applied to the International Court of Justice.
Held – The French argued that the court lacked jurisdiction. They also published a public statement that they no longer needed atmospheric testing.
Australia and New Zealand were not satisfied with the public statement as nothing stopped France from changing their minds and continuing atmospheric nuclear testing. The International Court of Justice denied their second appeal saying that the French declaration has already achieved what Australia wanted, that is an end to nuclear testing.
On the question of the reliability of the French statement, the court relied on the doctrine of Pacta Sunt Servanda.
In the end, the French did stop the atmospheric testing, but they continued underground testing as long as 1996 causing extreme harm to the geology of Australia.
4. Erga Omnes
The concept of erga omnes obligations refers to specifically determined obligations that states have towards the international community as a whole. It has origins dating as far back as Roman law and is used to describe obligations or rights towards all.
The concept is very important because in today’s structure of international society, composed of independent entities giving rise, as a rule, to legal relations on a consensual basis, erga omnes obligations can further enable the International Court of Justice to go beyond reciprocal relations among states based on consent in further developing international law on the basis of a natural law approach. By its very nature, this affects the freedom of state consent and the sovereignty of states.
ICJ’s jurisprudence is inconclusive. To be sure, it has expressly recognized a number of narrowly defined examples of obligations erga omnes, like the prohibitions against aggression, slavery, racial discrimination, genocide and right of peoples to self-determination, which is the latest update to this doctrine.
5. Jus Cogens
The jus cogens concerns principles of law considered universal and superior, and it means the principles which form the norms of international law that cannot be set aside.
Case law – Germany v. Italy
Between 2004 and 2008, Italian courts had issued a number of judgments in which plaintiffs, victims of war crimes and crimes against humanity committed by the German Reich during WWII, were awarded damages against Germany.
Ultimately, in 2008, Germany filed an application instituting proceedings against Italy before the International Court of Justice (ICJ), arguing that “Italian judicial bodies have repeatedly disregarded the jurisdictional immunity of Germany as a sovereign State”, and thus violating international law. Italy disagreed, stating that the underlying acts were violations of jus cogens and therefore, gave it the right to strip Germany from its immunity.
The Court rejected Italy’s claims and fully agreed with Germany. State immunity is part of customary international law, and the fact that the underlying acts were violations of jus cogens did not deprive Germany from its jurisdictional immunity. Importantly, the Court notes that while the current judgment confirms jurisdictional immunity of states, this does not in any way alter the possibility to hold individuals criminally responsible for certain acts.
6. Actori Incubit Onus Probandi
It is a legal maxim in Latin which means “the burden of proof is on the plaintiff.” Every plaintiff at law or complainant at equity must show a good title or claim before s/he can prevail in his suit.
Case: Republic of Guinea v. Democratic Republic of Congo
The case of Mr. Diallo concerns a claim of diplomatic protection brought by the Republic of Guinea against the Democratic Republic of Congo before the International Court of Justice. Guinea alleged that the DRC had violated the rights of Mr. Diallo, a Guinean citizen, by arresting and subsequently expelling him from its territory. ICJ decided on the violation of Mr. Diallo’s rights as an individual and as a shareholder in two Congolese companies.
On the merits, the Court found the DRC responsible for the violation of various provisions of international human rights treaties and of the 1963 Convention on Consular Relations. The Court found no violation of Mr. Diallo’s rights. On 19 June 2012, the Court decided on the compensation owed by the DRC to Guinea. This was the second time the ICJ decided on damages, and the first time it decided on damages in a case dealing with human rights violations.
7. Opinio Juris
An essential element of custom, it’s one of the four sources of international law as outlined in the Statute of the International Court of Justice. Opinio Juris requires that custom should be regarded as state practice amounting to a legal obligation, which distinguishes it from mere usage.
The Paquete Habana Case, 1900
The United States imposed a blockade of Cuba and declared war against Spain. While they were out to sea, fishing along the coast of Cuba and near Yucatan, two Spanish vessels engaged in fishing off the coast of Cuba were captured by blockading squadrons.
The fishing vessels had no knowledge of the existence of the war, or of any blockade. They had no arms or ammunition on board, and made no attempt to run the blockade after they knew of its existence. When the vessels returned with their catches of fresh fish, they were seized and a libel of condemnation of each vessel as a prize of war was filed against the vessel in court. The district court entered a final decree of condemnation and public sale at auction. Claimants appealed.
The issue that was raised was- whether it was proper for the court to issue a decree of condemnation and auction the fishing vessels?
Held – The Supreme Court ruled that, under the law of nations, in each case the capture was unlawful and without probable cause. It was a rule of international law that coast fishing vessels, pursuing their vocation of catching and bringing in fresh fish, was exempt, with their cargoes and crews, from capture as prize of war. Thus, the decrees condemning the vessels were reversed and, in each case, it was ordered that the proceeds of the sales of each vessel and cargo be restored to the respective claimant, with compensatory damages and costs.
8. Doctrine of Non-refoulement
Under the International human rights law, this doctrine guarantees that no one should be returned to a country where they would face torture, cruelty and inhuman treatment or punishment based on their race, gender, caste, religion, nationality, etc.
This doctrine is only applicable in respect of the country of origin and not to any country where a person has reason to fear prosecution.
Case: India’s case on the deportation of Rohingya Muslim refugees from Myanmar, 2018
Indian SC violated the principle of non- refoulement as it refused to stay the deportation of seven Rohingya Muslim refugees from India to Myanmar. These men were arrested in 2012 for entering the country illegally, without any valid documents. They were detained for 3 months at first and then additionally for 6 years.
ICJ took cognizance of this situation by commencing a preliminary examination and India was criticized heavily for its racist behaviour towards Rohingya Muslim refugees. UNHCR said that India has violated the well establish the international principle of non- refoulement.
Many said that India has ratified ICCPR, and must therefore take active steps to fulfil its treaty obligations. India is in breach of several international human rights and treaties.
9. Uti Possidetis
It is a principle in international law that territory and other property remains with its possessor at the end of a conflict unless otherwise provided by the treaty. This principle enables a belligerent party to claim territory that it has acquired by war. This doctrine originated from Roman law.
Originally ICJ applied this doctrine to establish the boundaries of decolonized territories in Latin America, and then it has become a rule of wider application, notably in the colonies of Africa and Asia.
 Oxford Public International Law, Available Here
Paquete Habana v. United States 175 U.S. 677 20 S. Ct. 290; 44 L. Ed. 320; 1900 U.S.
Mohammad Salimullah v. Union of India, WP (C) 793/2017