This page is about problems that may arise from evidence of travel to or visa for certain territories. For the issue of visas in general see Visa
It is a sad reality that diplomatic conflicts and wars often adversely affect travel. While most of those issues are more or less temporary, some have plagued travelers for decades, and don't show any signs of going away soon. The reason for being denied admission is usually related to travel to a place that is a) not recognized as a country by your national government, or another country that it borders b) has some sort of territorial dispute with the other country c) has sanctions put in place by the other country (see Americans in Cuba for the most long-lived example). There are some work-arounds for most of those cases, which will be discussed below. Note that none of the following constitutes legal advice and Wikivoyage assumes no responsibility for the consequences of inaccurate or potentially outdated information.
Israel and Arab/Muslim countries
While the state of Israel is a member of the United Nations, and has been at peace with both Jordan and Egypt for four decades now (and travel between these three is no problem), several Muslim-majority and Arab nations do not recognize the existence of the state of Israel and deny admission to anybody who is Israeli or shows any evidence of having been to Israel (e.g. visa (stamps) in a passport).
Countries that are known to deny entry to travelers who have been to Israel
Ways to avoid proof of having been to Israel
Before, avoiding the Israeli stamp was by having the stamp on a separate sheet of paper or by using an additional passport for those critical Islamic countries.
However, this is no longer a problem since in most cases Israeli passport control no longer stamps visitors' passports. Under a new system, visitors entering Israel are given special entry (and also exit) cards separate from the passport by the passport control. This is true for at least Ben Gurion International Airport and the Aqaba/Eilat border crossing, and may also apply to all other border crossings from Jordan and from Sinai (Egypt) to Israel.
Though border officials at Ovda International Airport (IATA: VDA) (marketed by no frills airlines as being close to Eilat) still stamps passports. So, ask the border guards to use a separate piece of paper.
Problems with overland entry/exit to/from Israel
Unfortunately, most countries that ban travel to Israel take a stamp from one of the border crossings in Jordan or Egypt that offers entry to Israel (or even just exit from Jordan/Egypt) as proof of travel to Israel. This is especially true for the specific land border exit stamps, as there is nowhere else that you could enter besides Israel. Therefore, you should try to get stamped for this border crossing in some other way than using the passport you intend to use for travel to countries that deny entry. While many border officials know of this issue, and will accommodate travelers by stamping a piece of paper instead, you may be unlucky on the Jordan or Egypt side.
In this case, you'll have to apply for a second passport, which allows you to have a stamp of any neighbouring countries or even Israel in one passport and travel to Muslim countries with another one. (Inquire at your own embassy.)
Note, customs of Muslim countries might also check for luggage stickers (or their residue) from Israel or neighbouring countries on your suitcase or the back(s) of your passport. So, remove any leftovers or signs of them.
Getting more than one passport
While this may be illegal or subject to some conditions (frequent travel, business travel), in some countries it is easily arranged for a nominal fee (usually not more expensive than your first passport) and perfectly legal to have more than one valid passport at a time in other countries. If you are unsure whether your country allows that, go to the place that issued your passport and ask well in advance of your Israel trip, as issuing a passport can take months.
United States of America and Arab/Muslim countries
Flying while Muslim is problematic in the US. In particular, under new rules passed in 2015, travelers who visited Iran, Iraq, Libya, Somalia, Sudan, Syria or Yemen on or after March 1 2011 are not eligible to enter under a Visa Waiver Program (VWP) commonly used by visitors to the US from Western Europe. They remain eligible to apply for a regular tourism or business visa – at the expense of more cost and hassle than with the Visa Waiver program.
Under a 2017 executive order, citizens of Iran, Iraq, Libya, Somalia, Sudan, Syria or Yemen have been turned away from the US despite holding valid US visas. In some cases, US landed permanent residents ("green card" holders), refugees (including local translators who assisted US forces during the 2003 invasion of Iraq) and dual citizens (who emigrated from one of the affected countries to a western nation other than the US) have been detained, turned away or placed on return flights to remove them from the USA.
As what can only be seen as a retaliatory gesture to the above, Iran now bans all US citizens from travel to Iran as of early 2017, despite their earlier policy of issuing tourism visas even to US citizens.
Entities with limited recognition
Some territories while de facto being under the control of one administration are considered by others (and often the international community) to be de jure under the administration of another country. While Wikivoyage takes no side on these issues, information as far as it relates to travel is provided below
Any sign indicating a previous visit to Nagorno Karabakh on your passport will get you permanently banned from Azerbaijan. Even an Armenian entry stamp, without actually setting foot in NK, may reportedly attract attention of the Azerbaijani border guards, resulting in an interrogation of the probability of an earlier visit to NK.
Since the official perception is that the island as a whole accessed to the European Union, entry stamps and visas of the Turkish Republic of Northern Cyprus (TRNC) are no longer an issue for later visits to Greece or the Republic of Cyprus-controlled half of the island, both EU members.
In accordance with this policy, arriving into the island through an entry point in the North is also no longer an issue while crossing the 'Green Line' into the South for EU citizens. Those travelling on US and Canadian passports are also reportedly spared from trouble. However, for Turkish citizens, an entry into the island through the North is still considered illegal by the Republic of Cyprus, and they aren't let through the intra-island border into the South.
Tourists can enter from Russia (Psou border crossing near Sochi) or from Georgia. However those who entered from Russia are considered by Georgian authorities to enter the country illegally and they could be prosecuted if they arrive in Georgia in the future.
Tourists can enter from North Ossetia in Russia, but, as is the case with Abkhazia they may subsequently face charges of trespassing the border illegally if they come to Georgia.
Serbia regards Kosovo as part of its territory, meaning that entering Kosovo through a border crossing that is not with Serbia is considered to be illegal entry under Serbian law. The official stance is that travellers with stamps or visas from Kosovo will be denied entry into Serbia though enforcement of this rule is sporadic. Entering Serbia through Kosovo, though, will be regarded as illegal entry, and would likely lead to you being arrested at Serbian immigration.
Although China regards Taiwan as part of its territory, there are no additional restrictions on foreigners whose passports contain Taiwanese stamps and visas. Taiwanese passports are, however, not valid for travel to China; you will need to obtain a separate Taiwan Compatriot Pass (台胞证) from the Chinese government for that.
Entering Crimea from Ukraine requires special permission of the Ukrainian government. Unless you hold a Ukrainian passport, the only way to Crimea is through Russia, and this generally does not leave any imprints in your passport other than a standard Russian visa with an entry stamp from Moscow, St. Petersburg, or some border town (it is not possible to travel directly to Crimea from abroad). You should, however, consider carefully when talking about your visit to Crimea. Ukrainian officials will likely consider this visit an offense and will ban you from entering their country. The U.S. and EU countries may do the same, although such cases have been unheard of to date.