Can a country start sending troops before declaring war? Can a country send invasion forces before an official declaration of war? So, if the US wants to invade France, can they, under international law, officially declare war by the time their aircraft carriers are already half way across the Atlantic, with clear orders to invade? Would it be different if they were going through their own territory, or a third country's, with permission?
This is a hard question, but a good question. The following relates to the issue from the perspective of the USA. You ask "...if the US wants to invade France, can they officially declare war by...", You have to make clear what you mean by "can". If you mean whether or not the US has the physical ability to do this, then of course they do. If you mean whether they are allowed, then this is another question. When you ask whether something is allowed, then you have to make reference to a moral code or a law that determines what is and what isn't allowed (legal). Normally each country will have laws that govern what is and what isn't legal for it's citizens to do. That hard part is how this applies to nations. There are two sides to the question. (1) What are the leaders allowed to do under their own laws, (the domestic laws) (2) what are countries as entities allowed to do? (the international laws) It sounds a bit like you are asking about the second thing. Here treaties govern what is and what isn't "legal". However, usually when someone breaks a law, other entities have the power to enforce that law and decide (judge) whether the person has broken the law. The question is: who decides whether a country has broken a treaty or an international law? And if a law is broken, what exactly can be done about it? This is hard to answer. Consider for example the conflict in Ukraine. Here many will argue that Russia has piratically invaded Ukraine. There should be war between the nations. But Russia denies this and other world powers have done little beyond imposing sanctions. This illustrates how hard it is to deal with this question. One question that comes up is what the purpose of international treaties are if a country can just violate them without much consequence. Pointing out two points about this. (1) If, for example, a president wants to convince his/her own congress that the country should engage in war, then it makes a stronger point if you can show how the country you want to engage has violated international trities. (2) After you have won a war, you might want to prosecute the leaders of the loosing power. Here you will stand stronger if you can make references to some international law that existed before the conflict started. This second point is illustrated in the Nuremberg principles. Here it was exactly stated that "Any person who commits an act which constitutes a crime under international law is responsible therefore and liable to punishment." According to the US constitution "Congress shall have power to ... declare War". That means that historically Congress has the power to decide where to wage war. The War Powers Resolution says, for example, that "The President in every possible instance shall consult with Congress before introducing United States Armed Forces into hostilities ...". Note that this doesn't mean that the US has do formally declare war before going to war. Example: The Iraq War was authorized by Congress, but there never was a formal declaration of war. It is interesting that you ask about the timing for when the declaration should be given. The Japanese wanted to deliver their declaration of war just before the attack on Pearl Harbor. But because of issues with decrypting the message from Japan to the Japanese Embassy, the declaration wasn't delivered until after the attack. Remember also in all of this that the winning party to a conflict, usually decides what was and wasn't legal! One good reference for more on all of this is the report by the Congressional Research Service called "Declarations of War and Authorizations for the Use of Military Force: Historical Background and Legal Implications". Here you can see more on what I have tried (and failed?) to say above.
On the first page of https://www.gov.uk/duty-free-goods/overview you find: "You can bring some goods from abroad without having to pay UK tax or ‘duty’ (customs charges), as long as they’re for your own use." And on the next page "... will use them yourself or give them away as a gift". This clearly doesn't cover anything that you bring into the UK to sell it. And a bit further on the site it says "You must tell customs (known as ‘declaring’) on arrival in the UK if you have goods: ... that you plan to sell". Since you are asking here, it is quite obvious that you intend to sell :-) You said "e.g. say this guys comes every month from Russia to France for unrelated business, each time he fills up his luggage with the maximum allowed amount of goods that comply with custom laws". If this guy sells the stuff to you, then the maximum allowed amount of goods to import without paying taxes is zero. Once he is willing to pay taxes, there is no limit. Obviously regularly importing things from Russia or France and selling in the UK at a profit means that he is running a business in the UK, which means he would have to register a business, pay corporation taxes etc. It's all a matter of degree; for small amounts nobody cares (up to some limit you don't need to register a business or pay taxes; I don't know the details).
Yes. Arbitration is a private, voluntary process, so two parties can agree to arbitrate virtually any dispute. However, because it is voluntary, Country B generally has no obligation to participate in the process once you've inititated it.
This is tied up in the concept of sovereignty - nation states have control over their territories and citizens and they recognise the right of other nation states to do likewise. The USA, China (Hong Kong) and Panama are all sovereign states, they each decide what the law is within their own territory and they can’t tell each other what to do; they can ask, however, that’s what diplomats do. If a HK domiciled company provides HK based servers then they have to comply with HK law irrespective of where their customers are located. The USA could pass a law requiring US companies (like ISPs) to keep logs of traffic to and from HK servers but they cannot force a HK company to do anything, unless and until it operates in a place where the US has jurisdiction which means both the right and the ability to enforce their law.
The Iran lawsuit depended on a statute directed against Iran, not applicable to the Chinese government. The Alien Tort Claims Act, which gives US federal courts original jurisdiction for torts "committed in violation of the law of nations or a treaty of the United States", neither of which is likely to describe the allegation against the Chinese government. The Chinese government probably would not entertain such a suit, and US courts do not have jurisdiction. It is also unclear how much US property is owned by the government of the PRC which could be seized: reports simply talk about "owned by the Chinese" without distinguishing government vs. private ownership.
Is it legal to cross the channel in a private rowing boat or sailing boat? Yes. Can he land on any old beach, or does he have to go to a port to identify himself? It depends. Who is on board, where did they come from, what have they got in their pockets, etc. This may be helpful Notice 8: sailing your pleasure craft to and from the UK When arriving direct from a country outside the EU (the Channel Islands are regarded as outside the EU for this purpose), you must phone the National Yacht line on Telephone: 0845 723 1110. You will need to inform the Yachtline if any of the following apply: etc etc Since France is in the EU, whether you need to phone the NYL to report your departure from the UK depends on a complicated bunch of conditions you can read in the link above. Maybe the French have similar rules? The RYA say Some countries specify ports of entry (ports where one may lawfully enter a country), which should be used by a vessel arriving from abroad. It is often a requirement that you proceed directly to such a port of entry on entering territorial waters. A vessel arriving in a country from outside its customs territory should fly the Q flag until it has been given clearance from the authorities. Even once clearance has been given, some countries may ask to inspect the vessel’s papers periodically, for example at each port of call. and so on.
The constitution has the "Treaty Clause" (article II, section 2) which states that the president "shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur". There is no legal concept of "signing" a treaty in the US, and only ratification counts. It is unclear what limits there are to enforcement of treaties in lieu of statutory enactment. Medellín v. Texas held that While a treaty may constitute an international commitment, it is not binding domestic law unless Congress has enacted statutes implementing it or the treaty itself conveys an intention that it be “self-executing” and is ratified on that basis This was a reversal of prior trends going back to Ware v. Hylton based on the Supremacy Clause, that all Treaties … which shall be made … under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby But Medellín doesn't mean "never": you have to "parse a treaty's text to determine if it is self-executing", which is a different ball of wax.
Sharing intelligence is an act of war inconsistent with neutrality However, so is supplying arms to a belligerent. A neutral State must never assist a party to the armed conflict, in particular it must not supply warships, ammunition or other war materials directly or indirectly to a belligerent power, but otherwise its trade with the belligerent States remains unaffected. However, it’s not a war crime.