Spain forms the eastern boundary of Portugal.
The country is Eritrea, and it's eastern boundary is formed by the Red Sea.
Spain is the only country to border Portugal to the east.
Spain
Sudan is the most eastern country to Eritrea.
The eastern boundary of France and the western boundary of Hungary are separated by approximately 800 miles (about 1,290 kilometers) in a straight line. This distance can vary depending on the specific points chosen for measurement on both boundaries. The actual travel distance may be longer due to the routes taken.
In most cases the eastern boundary of something will touch the western boundary of the next door something. Where "something" can be a state, province, country, estate, ocean, lake river, Earth plate etc.
Polish Corridor
The southern half of the eastern boundary of Alabama is the Chattahoochee River.
Eastern U.S.A.
None. The eastern edge of Russia is a sea-coast. The nearest country to it is the USA; Alaska is separated from it by the Bering straight.
The natural boundary that separated Fort Duquesne from the eastern seaboard was the Appalachian Mountains. This mountain range served as a significant geographical barrier, influencing travel and trade routes during the colonial period. Fort Duquesne, located at the confluence of the Allegheny and Monongahela rivers, was strategically positioned to control access to the interior of North America while being isolated from the coastal areas.